I don't know what the problem is. Please tell me.
 

I'm doing free motion quilting. It all looked good from the top. Then I noticed the back side has the stitching really inconsistent. Every few stitches has another few that look like a straight line. I have tried rethreading the machine, reinstalling the foot, playing with the tension, different needle. Any clues?

What kind of machine are you using? Not all machines will do fmq well. Those with horizontal bobbins usually don't stitch well in fmq, because the thread has to take a turn at a right angle and it just doesn't time right. Sounds to me like your stitches are skipping. Are you pulling or allowing the weight of the quit to pull on it as the machine stitches? That can be a problem too.

What you're describing sounds like what we call "eyelashing". Here are some resources:

http://freemotionquilting.blogspot.c...-or-speed.html
http://www.quiltersclubofamerica.com...irdsnests.aspx
http://www.generations-quilt-pattern...eshooting.html
http://www.quiltingboard.com/main-f1...s-t179288.html

I think you may be on to something. It seems to be in one particular direction. I try not to let it pull, but really all I have is an inexpensive brother machine sitting on a desk. Still, I've quilted other things that didn't do this. The only other thing is this is the only time I've used a polyester batting. Would that matter. I don't think I like it anyway and won't use one again.

I seriously doubt if it is the batting causing your problems. I use poly all the time and love it. How did you baste the sandwich? If you used washable glue maybe the glue isn't quite dry or if you spray basted maybe you used a bit too much? Have you tried a larger size needle, like a 16? Or maybe just a new needle? Those are all things that have caused skipped stitches for me.

I've started using a product called Sewer's Aid. It is a silicone thread lubricant. Just a few drops on the spool of thread and things go much better. It's worth a try.

Are you moving the quilt at a consistent speed? If it's only in one direction, could you be moving the quilt faster in that direction?

I get best results if I turn the speed down a little - both the machine's speed, and my own! I tend to try to sew as fast as possible but that doesn't work out so well for FMQ.

I had a problem recently with skipped stitches on a project I was working on. I rethreaded the machine, changed thread, needle etc and the problem still existed. I finally discovered it was the fabric. I was using batik and there were four colors of fabric. I bought all the fabric at the same time from the same place but there was something about the blue one that was giving my and my machine the fritzes. The other colors were fine. I finally changed the tension when I was sewing on the blue and that did the trick so check to see if it is just happening on a particular fabric too.

9 times out of 10, issues on the underside is the quilter.......eyelashing is when you are moving your fabric faster than your speed on your machine. Try slowing down, particularly on curves, or speed up if you want to keep your fabric moving faster. There has to be a perfect speed....with your fabric motion and the speed of your machine. This is why it takes a lot of practice. Do some small practice sandwiches and see how your hand motion and machine speed can be matched a little better. Above all, don't get discouraged....BTW those practice squares can be made into pot holders! Just slow down your fabric a little on those curves....and you should be just fine. You didn't say if you were new to FMQ. Also, make sure your fabric is moving freely...sometimes holding on to the fabric too tightly causes you to push harder, which then makes your fabric move to fast. Also, don't mess with your tension for FMQ. Many quilters think that it is their bobbin tension....sew a straight line, if your tension is OK, then it is your speed....slow your hands down or speed up your machine on those curves. If you have skipped stitches it is because you have moved your fabric faster than your machine's ability to do the lock stitch with the bobbin thread.

Do you have your feed dogs down?...when I have the straight lines on the back it usually means it's feeding itself the way it wants to go...I have a hard time remembering when I'm changing back and forth..