It can also be the top tension being too tight. Make sure the top thread is running freely, if it gets caught at all often the bottom shows alot.
Lower the embroidery foot, to the thichkness of the fabric being sewn.
hope by now you are already on your way to getting it done without anymore problems but just in case here is what i have noticed with my machine...
I have found that it depends on what thickness my fabric and stabilizers used are so i also agree with the comments about tension settings and needle. it always is a good idea to clean the fluff out and do a test stitch of it so you don't ruin the "real" fabric it is going on....
perhaps you have to change the tension, when you see the bobbinthread on top than reduce the tension of the embroiderythread. and when there's too much of the embroiderythread on the back then give the embr, thread more tension.But \I agree with tinfriend, you can also try another needle or clean the bobbinplace. Hope it helps you
Definitely use a NEW Needle!
If you are using Pre-wound bobbins then tighten your tension at the bottom on your bobbin holder (shuttle).
Also check for any fluff and clean out your machine where your bobbin fits in. Check that you have no 'nicks' on your 'hook' - this is what catches your thread in your machine. You can use 'water paper' to smooth it out. OR take it to your dealer to have it smoothed out!
Also clean out top tension plate - where you adjust your top tension & you thread your machine!
This should solve your problem!
*4U!