Someone asked about getting good tensions and really, I think most of our sewing machine troubles come from tensions being unbalanced.
They don't teach sewing in most schools nowadays, and a lot of young women aren't lucky enough to have grandparents to teach them like I did, so maybe I can stand in momentarily and be a gramma for someone learning to sew.
Without balanced tensions, your sewing machine may not sew a nice-looking, strong and reliable seam.
It takes some practice and some trial and error, but if you can learn how to balance the tensions between bobbin and needle threads, you will solve about 90% of all your sewist woes. :)
This method won't work for everyone on every machine - if you have an expensive computerized machine, it might have a tensioning system that isn't user-adjustable or even user-friendly.
You may have been told by your sewing machine guy/gal not to ever *touch* the bobbin tension. If you have one of those kinds of machines, please don't follow these instructions. I don't know anything about those machines - I sew mostly on vintage machines partly for this exact reason - I can adjust all of them myself whenever I want to.
Another benefit is that you can sew with wildly dissimilar threads in the needle and bobbin. I promise that it isn't as hard as it might seem at first - it just takes practice.
1. First, match the needle to the thread that you're using in the needle, regardless of what thread you're using in the bobbin.
If your needle is too small or too large for the thread that you're using on top, the needle cannot produce the loop for the hook or shuttle to catch. Skipped stitches or no stitches at all.
2. Make sure the needle is straight, sharp and correctly seated.
3. Thread your machine properly. If you have more than one machine or it's been a while since you sewed on this machine, you may have to look at the manual. Your manual is your best friend.
4. Make sure that the thread path is clean and unobstructed by a bit of lint or thread or anything.
5. Now, fill the proper bobbin for this machine and keep the thread flowing onto the bobbin smoothly - if it looks like a bowl of spaghetti when it's full, you're not going to get a consistently good stitch. Slow down and make sure the thread lies neatly on the bobbin as it fills.
6. If you have a removeable bobbin case, it will be easier to adjust the bobbin tension to "just right," but in any case, it takes a little bit of practice.
Insert the bobbin properly into the bobbin case - go back to the manual if necessary - if the thread is off-winding in the wrong direction, it will mess with your tension and the machine may not even form stitches.
7. Holding the bobbin case in a way that allows the bobbin to spin freely, (or with the bobbin properly threaded in the case, if it's a horizontal/drop-in style) pull the thread that's extruding from it. It should have some resistance - you shouldn't have to tug on the thread, it should unwind smoothly and easily, but it will not feel like you're unwinding a bobbin that's just sitting on a spool pin, either.
8. Now if your bobbin case is horizontal, you're going to have to skip this "yo-yo" test and go on to the sew test.
If your bobbin case is removable, then with the bobbin case lying in the palm of your hand, lift it (slowly) by the thread that's extruding from the bobbin case. Give the thread a tiny little dip, the motion you would use if you're playing with a tiny little yo-yo. The bobbin case should slide down the thread a little bit and stop. If you do this over a tray or shoe-box lid, you will do less chasing of bobbins and bobbin cases while you experiment.
If the thread just unwinds as you try to lift it and the bobbin case stays in your hand, then there is not enough tension on the bobbin thread. Consult your manual as to how to apply more tension. For all of my machines, it is a simple clockwise turn of the tiny screw that tightens the tension spring on the bobbin case. (And if you *don't* do this step over a tray or a shoebox lid, you will eventually regret it.) There may be another screw on the bobbin case - that will be the one that holds the spring on the case and should always be tight - look at your manual to see which one adjusts the tension end of the spring.
Test the tension again. Tighten or loosen the spring if necessary and test again. If you're nervous about this, you can start with just turn a quarter-turn at a time. For fine-tuning, maybe only a few degrees at a time.
If you accidentally unscrew all the way, I've found that the easiest way to re-set it is to lick my finger and touch it to the slotted end of the screw (which is lying right there in the shoe box lid, right??? ;) ) then touch the screw to the hole in the spring where it belongs, press down and turn the bobbin case counterclockwise against the screw. Most of the time, it will seat itself.
9. Okay, so now the bobbin tension is correct for the thread that you're using in the bobbin and your needle thread is ready to go, too.
Put some fabric of the type in your project under the presser foot, lower the presser foot, bring up the bobbin thread and hold onto both of them and sew a few inches of seam.
Remove the piece and look closely at the seam. The seam should be smooth and have no puckering and both sides should have flat, even stitches.
10. Troubleshooting the seam - if there are no stitches on top, just a line of thread with little loops holding it to the surface, the top tension is too tight. Loosen it a half-turn of the tension knob.
Sew a few more inches and examine the seam. Better? If not, loosen the tension another quarter or half a turn at a time until you're getting a good seam.
If the stitches look good on the top but the bobbin thread is lying flat with loops of upper thread around it, then there isn't enough tension on the needle thread. Tighten it by a half or quarter-turn of the knob.
Sew a few more inches and see if it's better or if you need to go just a little tighter or if maybe you've gone just a bit too tight.
If the stitching looks good and even on both sides but the fabric is puckered, the tensions are too tight on both the needle and the bobbin threads.
Go back to the bobbin thread and loosen it just a bit at a time, testing it and balancing the tension on the needle thread as you go.
When you get a seam that looks smooth and even on both sides and the fabric lies flat along the seam with no puckering, you've just balanced the tensions correctly for that combination of fabric and threads.
If you change anything in that combination, you may have to tweak one or both tensions.
This *sounds* very complicated, but honestly, it's taken a lot longer to type it than it takes to do it. :)
It's only a few seconds or maybe a few minutes when you start to sew and you're ready to go. Once you learn how to do this, your frustration level is going to go way down - you'll know how to get the best seams out of your machine and you will make all these adjustments automatically, without even thinking about them. They're very easy once you've tried them a few times and you'll be a better seamstress for it. :)
I hope that helps someone, somewhere. You can do this! :)