If this is the same as the Sweet Sixteen, then it could easily be the same as my problem, since mine is an HQ fusion.
When you take the bobbin case out, you'll see a metal bar across the back, with openings top and bottom. The center is where the shaft connects. There is a small gap back there and that is where the lint built up. I was shocked at how much I cleaned out of that area as it was not visible at all.