If your bobbin tension has become too tight (maybe some fiber got caught in there) that can make the top thread look too loose, too.
I don't know if this applies, but I had a serger where I could not get the tension on one of the threads to be tight enough no matter what I did. Turns out that it needed a repair, and I was told that there was "a little weld" in the tension discs that had apparently come undone.