Threading your serger correctly is really key. On my machine, I first thread the lower looper, then the upper looper, then the needles (on my machine it doesn't seem to matter which needle I thread first, but on some machines it does). When you have it threaded, pull the threads under the presser foot and at the 11:00 position (kinda crosswise across the machine bed). On my machine, the upper looper thread must lie over the lower looper thread.

Make sure your needles are in correctly, with the flat part of the needle shank facing away from you on most, if not all, machines. Do you have the needles inserted all the way up into the needle bar? A hinged dental mirror is very helpful to make sure. My machine must have a number 90 or higher in the front needles, any size in the back.

If you have a two-thread option, make sure when you do 3-, 4- or 5-thread overlock, that the two-thread converter is disengaged.

Just some thoughts. Good luck!

Mary

Mikee, to Mary's suggestions I would also add opening it up and cleaning the lint out of it really well.

My old serger acted like you describe and I discovered the needles weren't inserted all the way.

After you do everything Mary suggested,if it still doesn't work,turn all your tensions to 0,get out your manual and set the whole thing up againg starting from scratch.
Kathi

If you are sewing a knit fabric, sometimes ball point or stretch needles may be the key. You could also be feeding the fabric incorrectly.

I would go back through the manual and follow instructions step by painful step. There are always little details missed that come back to bite you later.

take a look at the timing theres a tutorial on that in the link

Here is a link that might be useful: serger repair info

what is the solution to this problem?