Whenever I sew on medium weight material, the bottle thread is all jumbled up. I've rethreaded the machine several times and have changed the tension and nothing works..

sounds like the timing is off and will need a service call to get it reset. your local fabric store should know who you can call.

Something here may help:

How to Sew Pin Tucks Threads

Try removing the top threads from the machine. ALWAYS RAISE the presser foot and rethread both threads. Could be possible your threads were not seated in the tension disks.

the thread that you changed to is a lighter weight thread & the top tension must
be adjusted to work well with the thread & fabric
needles & thread have to work togeather with the fabric,, & the tension needs
to be check every time thread & needles & fabric are changed

Check YouTube for some great video advice. I was checking for tension issues and she addressed "bunching" issues too. Recommended teflon bobbin washers!

it is a needle thread tension problem
as the tension wheel makes no difference , that indicates that the thread is not between the tension plates but behind them
you can set or check the tension by simply pulling on the thread before you thread the needle
proper tension is when there is a resistance to pulling the thread by hand

Firstly have you interfaced the material that you are buttonholing on? You will never get a good buttonhole otherwise. Also start each new project with a new needle matching the size to the weight of fabric. Good guide available here http://sewing.about.com/od/sewingmachineindex/ig/Sewing-Machine-Needles/

Top thread breaking is usually because the tension is too tight or the thread is catching somewhere in the thread path from the spool to the needle. Also check that the needle doesn't have a burr on it somewhere, or is blunt.

Thread snarls underneath the fabric is usually caused by top thread tension issues. You will need to rethread top and bottom and CHECK that the thread is in tension correctly. So lower the pressure foot and pull the top thread just before the needle - does it feel hard to pull? If not, then the thread is not in tension. Rethread top again, checking the tension dial is set to 5 and the thread is correctly pulled between the tension discs. And always hold the two thread tails at the back of the sewing area as you start each new seam to avoid these being pulled down into the bobbin area and getting jammed up.

So, rethread, test sew and get the top tension sorted, a nice straight stitch happening, then try the buttonholing sequence again.

You would do best with a heavy weight needle. You also might want to use Upholstery thread rather than normal weight thread. Depending on the machine, you probably need to change the tension. Some high end machines do this automatically. Your best bet is to try a test run on scrap material to get the right needle, stitch, thread combination. Good luck.

the tension must be adjusted the the material you are working with. the thread also is a factor,play with both and adjst accordingly

Rethread the machine from the spool to the needle.All machines thread the same. TTN tension, take up, needle.
Beginning with the spool, guide the thread through the thread guides to the tension assembly. Pull the thread through the tension assembly and test it to make sure the tension is working correctly.
(To do this test, adjust your tension setting to normal or medium or 5 or something in the middle. Raise the presser foot and pull the thread through the tension. It should pull easily!! Lower the presser foot and pull the thread. It should pull noticeably harder.) If it passes this test, continue up through the take up lever,then down through the thread guides to the needle. Make sure your needle is fully inserted and turned the correct way.
You need to be sure your tension is working correctly. Adjust the tension to 9 and pull the thread. It should be hard to pull. adjust the tension to 2 and it should pull much easier. If it works as described, then adjust the tension to 4 and your stitch length to 2.5 and try it on some medium weight fabric.
If you are sewing on light weight fabric set your stitch length to 1.5 or 2. You may even have to reduce the tension to less than 4. You can reduce the tension until you see loops on the bottom of the fabric at which point you should tighten the tension to remove the loops.
sewman7

Switch back to regular thread. Upholstery thread is used for seams that will get lots of stress (as in upholstery; although I use regular thread in upholstery and have no problems). For draperies, regular thread is fine.