Did you thread it in sequence as instructed? Have you replaced that needle? I know some sergers show one needle longer, but are you sure you have it in the correct needle hole? I had a serger once (Elna) that used a setup like a twin needle (connected) but one was longer. I think the later models are adapted for drop on one side. I would suggest cking the manual to be sure it is installed in right place...and rethread the loopers. I believe there is a step being missed somewhere.

Unscrew the screw holding the right needle and see if it has been inserted *all the way* in. In some machines, one needle sits a little higher than the other one.

Yes, I made sure to thread it in sequence and I put new needles in too. One of them is a bit higher than the other, I double checked and they are in correctly. I think it must be a tension issue with the right needle, I have to play around with it some more. Once I get the tension right, I love my serger, it's such a time saver and makes a neat finished edge. But trying to get all 4 threads in proper tension takes a bit of effort.

I had the same problem with a Singer serger that I used for years. In the beginning, and for many years, no problem. Toward the end of it's much used life, a) the thread would slip out of the first tension guide (where the thread enters the machine just as it comes off the thread spool) or wouldn't totally engage in the tension finger, and/or b) the left looper thread would slip out of the guide underneath the throat plate...very difficult to see in my machine and it took quite a while to figure out what was happening. I suggest you check all your thread guides and make sure the threads are running through the guides as directed in your manual.

You can more easily see where tension needs to be adjusted by threading with different colors of thread. You can trace the problem by the color.

Good idea!