Welcome to the Quilting Board!
The one other thing to check is your bobbin. If the thread isn't coming off the bobbin correctly it'll make a huge mess and not sew correctly. If it's not thru the little slot area it's not under tension.
This thread certainly explains why my Janome 6600 doesn't match the instruction book that came with it. I only have one plate (the book doesn't even mention two, AND it the pictures don't look like my machine! yes I bought it new). The nearest repair shop is a long ways away and the repair man has shown DH and me how to care for/clean. But when it was new I found myself calling him every month or two. He would always say 1) try a new needle; check the size; 2) check the bobbin area; 3) rethread (DON'T just check it, rethread!); 4) calm down... go get a glass of wine; 5) if none of this works, call me in the morning... Love that man! There is obviously more than one Janome 6600!
Had that problem with my 7700 and had to take it to the repair person cant remember what was wrong but had to replace something
A time to tear, And a time to sew;
A time to keep silence, And a time to speak;
I had a similar problem when I got my Janome. It turns out that the bobbin wasn 't clicked in place correctly. See if that could be your problem.