I own this exact machine. First, there is not a way to adjust the bobbin tension thread to my knowledge. There is not a typical screw and bobbin holder. The bobbin drops into the base. I have learned that if you run a stitch off of your material, the bobbin thread "jumps" but still looks like it is in place. It is necessary to pull the bobbin out and reinsert. I also know that it is better to take a scrap and start on that and then feed your actual quilting piece into the machine OR hold BOTH threads back when you first start sewing. I have also had problems with deco stitches....I am going a long fine then all of a sudden the stitches start messing up in the middle of a quilting project.*** Not a great machine.***

rethread.

In answer to the question about threading the machine, it is best to have the presser foot up while threading the machine as it then goes into the tension disks to get the correct tension. In the end, lower the prsser foot to insert the thread into the needle.

If you changed for a poly thread to a cotton on top, I would guess you missed something on the top when you re-threaded or the auto-tensioner on top is not happy about the sudden change from a slick poly to a dry, possibly linty, cotton. Re-thread the top with the presser foot up and pay close attention that the thread snaps into the tension disks properly. Then you might want to adjust the top tension. On my machines, it is numbered and the higher the number--the tighter the tension. Cotton usually takes less tension than poly but maybe not in your case sense the bobbin thread doesn't seem to be getting picked up which is an indication of not enough tension coming from the top to pull it up.

I used to have that same machine...I found that it "liked" poly thread much better than cotton...I used Coats & Clark trilobal polyester thread, and rarely had an issue with it...here is a pic of the thread for reference:
http://www.coatsandclark.com/product...ery+thread.htm
Good luck to you,
Kif

Did you change the needle too, the needle may not like that thread.

If your machine is in the "auto tension" mode, over-ride it. You will have better luck if you can adjust the tension manually. Your machine has no idea what thread is in it, but different types and brands of threads could possible need tension adjustments.
Ricci

Success! I rethreaded, again, and gradually increased the tension on the top thread until it looked like it should on both sides....no more eyelashing and the bottom thread isn't just laying there....so much stress over such an "easy" fix....LOL