I have a Janome 6600 which I now love. But the first six months of sewing with it made me crazy. The tension was terrible most of the time but sometimes okay. The store where I bought it couldn't figure out what was wrong. Every time they tried it out, it was fine.
I've been sewing for years without trouble on my inexpensive mchines and was broken hearted that I had paid so much for this machine & it didn't work. I tried everything ...different threads, pre-wound bobbins, etc.
As it turned out, the fix was embarassingly simple. I was not always putting the bobbin in correctly. It has to go in with the bobbin thread unwinding counter-clockwise - just like the drawing on the bobbin cover plate. Such a simple thing, but it corrected the problem.