I bought 3 spools of thread from the same place, same manufacturer....same size, same needle (277 and #24) the white and black ran fine...the brown kept dropping stitches. It would be fine for say, 12" of a belt, then drop a stitch, ok again for another foot, then drop a stitch......I change to white and or black, flawless..back to brown, drop stitches....get the picture.
I asked the thread people if it could be the thread, they said they would replace the brown, the new spool runs perfectly, knock on computer desk......
I ruined a couple brown belts this way, there's only so much fixing and clean up one can do til it won't pass CQ anymore.
I'm wondering if the thread isn't wound right on the spool, it can kink at some point and cause this....I don't know. I just know , new spool, problem has gone away....