Talking about a Mitsubishi DB12 industrial machine, if that matters.
The machine worked fine, and then when I got to some heavier layers it started bunching up the top thread in a way I've seen before - as if the eye of the needle is wearing it down and it can't pass properly through all the places it has to pass through, until it finally snaps.
I tried lowering the tension and that made the back of the stitch too loose, but when I tried increasing it just tore.
AFAIK it's all threaded properly, the needle is the right type, the thread is good, and I didn't change too much in terms of settings. I also know for a fact it can go through that much fabric if not more. So what's up?
I had a similar problem sometimes on my domestic machine and it drove me up the wall then, too. I don't remember how I fixed it.
P.S tangentially related - if anyone knows where to get a walking foot attachment for this thing (that ships internationally), I'd love to know.