I know very little about sergers other than they can be picky devils.
My wife has two, a Brother 920D 4 head, and a Singer 14U285B 5 head and this is what I know about them:
A: If you have an operators manual read it and follow the directions to the letter.
If you don't have one, get one, and follow the instructions to the letter.
B: One thing I've been told and have seen is threading is critical. Not only must the thread follow the right paths, it has to be done in the right order.
C: Make sure they are cleaned inside. Sergers make more debris and lint than any 10 sewing machines. Especially under the feed dog / cutter area.
D: When my wife is working on them I leave the room.
Sorry I'm no help on this, but sergers are simply beyond my scope of ability.
Joe