There are only two possibilities: the top tension is too tight, or the bottom is too loose.
I'd try first setting the upper tension to some point that you think is ridiculously too loose, to see if that changes anything at all. If it doesn't, then go to the bobbin tension and try to tighten that.
Once upon a time I had a new (three-week-old) machine that I had to return for a fix in the bobbin tension mechanism. (That was before I realized that you can actually tweak that yourself.) And I've also had a serger that had problems with the upper tension that also had to go in for repairs. I was told there was a "weld" somewhere that had let go. (That I would not have been able to fix myself).
Good luck. It's so frustrating!