I can't tell you about other machines but I can tell you that the Gammill machines are extremely sensitive to the very tinyest little things. It can take hours to track down the source of a problem because it might be so small you need a magnifying glass to spot it. Before you decide to take a hatchet to the machine so you can toss it out the window.... here's one more thing to try when you are having lots of breaking thread or intermittent tension issues.
Take apart your tension assembly and check the felt disk. Is it dirty? This dirt puts drag on the tension disks which causes tension problems. Check the disk for lint and dirt inside the center hole. See how dirty mine was? Also, inside the center hole of the disk is a tiny, tiny little groove. It's where the two sides of the disk come together. Sometimes there is lint inside this that you can only get out by using the pointy end of a needle.
Maybe your machine doesn't look like the Gammill? Well basically all machines can be affected by lint in the tension assembly. Next time you can't figure out what's causing the problem; check this before getting angry at your machine. Oh, and if you are like me and realize that was the last one of your dozen felt disks..... it can be turned to the other side long enough for more to arrive in the mail.
(This is one of three posts I've been working on at the same time. All with different topics. The others may appear on the blog out of order because that's the way blogger works. Check below this one for the others as I get them finished.)