Tension - what is wrong?
If you take the time to work through the TENSION tutorial named Secret To Taming Your Serger ONE TIME, you will know how to set your own serger and correct your own tension settings forever. This helps you create a "stitch dictionary" for how your own machine produces balanced stitching:
Secret To Taming Your Serger
Adjust Presser Foot Pressure
DIBY's Serger Tension Guide
SergerPepper's Fabulous Serger and
Serger Tension Tutorial based on a Juki but great detailed photos
Threading Your Serger