I have a Janome 6600 and I had this problem until my dealer said that she cranked her tension (top) all the way up to 9 or a bit more. You may also be sewing too slow for the speed at which you are moving your quilt around. FMQ is a real art & the skills needed are gotten by practicing and practicing and pract . . . Get the picture? My first attempt went in the trash bin. Even now, I have a practice sandwich that I play with to warm up before working on the "real thing." It's like playing the piano - gotta practice those scales with proper fingering before you can play a sonata.
Good luck and have FUN. Remember to breathe and relax those shoulders. :P