In addition - you might check for burrs in the metal any where the thread passes. Dried oil & lint will do the same. Thread can break or just not pass smoothly causing the tension to be off. If you have some burrs you will need to polish them out. I have a machine that still makes tunnels or ditches under the zz stitches. I'm not sure what causes it on that machine. It can be very frustrating. A lot of it is touch, feel and experience. Those OSMGs do have experience. It can take working on a couple hundred machines to know what you are doing - even then there are new things...