I heard a tip recently when it comes to things like this and it was TNT. Many immediately think they have a tension issue when that is rarely the case. First check THREAD. Is your machine threaded correctly and are you using the same kind of thread you used when you weren't having any problems.
NEEDLE - is your needle dull, bent, inserted correctly. Are you using the right kind of needle for the material and the thread. In your case are you using the same kind of needle you used when you had success? I had a very frustrating time with one project with skipped stitches and such. It turned out to be the needle was not compatible with the blue batik that I was using. When I changed to a sharp needle from a universal needle the problem went away.
TENSION - if all else fails it could be the tension. I never have to change the tension on my Janome 8900. The only time I have is when I was doing gathering for a project and my instruction book that came with the machine told me to change the top tension to a certain point for that stitch.
Speed is a big factor in FMQ and stitch quality especially when going around curves.
Hope these tips help.