Sometimes people have taken apart the upper tension mechanism and re-assembled it incorrectly. You may need to hunt online for a manual that shows correct tension assembly and re-do it yourself. However, I'd try everything else first!
One thing that can affect upper tension is lint. Try "flossing" between the tension discs with *non-waxed* dental floss. A piece of lint or thread caught between the tension discs will keep them further apart, keeping tension too loose.
Also try looking for another thread guide up above that you might have missed. That can drastically affect tension.