I have a Janome 4800 and I fmq on it all the time. When I first started the stitches were too loose, I took it back to the lqs that I bought it at and they adjusted the bobbin tension and it's been perfect since. You do have to be careful when you adjust the bobbin tension though. Good luck and keep on trying. If you take it to the shop where you bought it, make sure to take some samples of what it is doing and squares to practice on while you are there.
Sue