I have had three problems with getting a serger to sew. One was the wrong needle. Mine needed a special slightly shorter needle. Next problem. Always take all thread out of the lower part. Then thread starting from the bottom. Something about the way they get wound around when you turn the wheel to thread the upper looper it seems. Next, We sewed a sample and checked which thread was not sewing properly. Took that tension assembly apart and the spring was not set correctly. Also lint in the bottom can cause problems. Have you tried sewing on the onesy with a piece of tissue paper or stabilizer. Or vary the width of the stitch. Or try a three-thread stitch.
Hope you can get it conquered. They work wonderfully when they work.