Make sure your ( top thread) is in the tension discs correctly-
I have found with my Husqvarna Viking sometimes it is the thread- after Trouble shooting all other things it could be if nothing has fixed it- I break down and change the thread- problem solved. My Viking simply will not work with some thread. And, it doesn’t matter if it’s the same brand I was using before the problem started. When all else fails try a different spool of thread. For me this has always worked. And I’ve been sewing on a Viking machine for 20 years.