I have an HQ16 and I use Superior So Fine on top as well as the bobbin. When I first got the machine I tried using the Bottom Line thread which is much finer in the bobbin but had tension issues. So Fine is 50wt and doesn't have a sheen and blends very well. Also check your needle size ...most of the time I use a 16 or 18. Batiks are a little more challenging to quilt so go for a finer needle (14) and lastly check for tiny bits of lint in the bobbin area as well as the tension discs. These are the things that worked for me after much trial and frustration. The folks at Handiquilter also have some short You tube videos on tension and cleaning your
machine. Check them out if you have the time and good luck!