If Hazeljane's idea doesn't work, make sure your feed dogs are down. Also, have you seen the Magic Bobbin Washers? Leah Day, a popular free motion quilter, said she couldn't work without these. They are supposed to end the thread loops and birds' nests on the bottom side of your quilt. Take a look at this site.
http://daystyledesigns.com/magicgeniewashers.htm
I bought some for my machine but haven't had a chance to try them...my machine is in for repairs. Good luck to you!