I have a Tiara. Make sure that your needle is seated all the way up. Also a very small adjustment in the position the eye of the needle faces makes a big difference, i.e. as in a clock face, have the needle at 5:30 or 6:00 position. Hint, insert a needle or pin in the eye of the needle to make those tiny adjustments. Sometimes just rethreading from the get-go is all it takes. Good luck. The Tiara is an awesome machine and once you get your issues figured out to will love it!

I also have the sweet 16 similar to Tara and the YouTube videos are great and go into the tension step by step.
i think that is your best bet. If that doesn't work for you, you will,probably have to have it serviced. Elsie

I would recommend watching TNT - Thread, Needles, Tension. https://www.handiquilter.com/videos/education/3/

Tira
 

I have had the same problem. The needle size you should use is a 16. Make sure that the cut out part of the needle is toward the back. Look in the book that came with the machine. I use that every time I change the needle just to make sure I put it in right. If you are not using a cone thread, try one and see if that helps. It took me a long time to get the tension right on my machine too. It just takes practice. Don't give up. It's really a lot of fun once you get everything right at the same time.

Were the bobbins your friend sold you with the machines empty or ones she prewound? If she prewound them, the winding tension may have been off. To check, take a small flat object (like a screwdriver) and push against the thread. Is it squishy? If so, the thread was wound too loosely and will give the problems you describe. HQ has an instructional video on how to use the bobbin winder and check the tension. Good luck. I know from personal experience how frustrating it can be to just want to play with your baby and end up having problems.

I don't have this machine but there are two things I would put forth along with what others suggested. Get some of those bobbin washers and try them out. Sometimes due to the high speed of these machines, the bobbin can end up spinning in the bobbin holder and cause problems. Also, I have a suggestion related to the problems you are having removing and inserting the bobbin. I bought a E Z Bob for my machine (Pfaff 1200). I too found it difficult to insert and remove the bobbin on that machine so I tried the E Z Bob and after a bit of practice, I found it to be much easier. My fingers seemed to be in the way or too short so this device helped. Here is a picture of what it looks like:
http://www.sewingmachinesplus.com/check-hh103.php
It's just a plastic rectangular piece with a hole in one end. I am not sure who sells them now.