I spent an entire day trying to solve a similar problem on my HQ16. I finally said,"Tomorrow is another day." and watched TV for the balance of the evening.
The next day,when I approached my machine, I saw that I had not elevated the thread on the 'high thread thing', so the thread was dragging and therefore not coming off the spool evenly. All was well after I re-threaded. Thank goodness. I can't see the spool on your machine so can't comment on whether the thread might be dragging because it is not threaded high enough to come off the spool evenly. Before that happened to me, I had no idea that could affect the tension like it did until I had the problem, so thought I'd mention it.
To set my upper tension, I turn the tension to its lowest setting. Then I sew a few inches, stop, check the back, adjust the tension to a tighter setting - then do the entire process over until the tension looks good to me. The back looks better with each turn of the knob, so I know I'm making progress. This system works well for me!