In order to understand the signficance of each thread on your overlock and coverstitch, use different color threads to make a sample.
I prefer working with a piece of heavy felt when I do samples like this. I find that it's just the right weight that feeds through beautifully and I only have to use one layer.
Color coding will enable you to understand how the stitch weaves together and when problem-solving you will know which thread needs adjusting. This is something that is more difficult to figure out when the threads are all the same color.
This is worth the time that you invest in setting it up. Keep the sample handy so when you need to make tension changes you will know exactly what to do.
I've meant to do this for so many years and wish I had done it sooner and saved myself alot of aggravation.
Also, included on my sample is information on the settings I used - sl (stitch length) and tensions.