..."It was because I was using the wrong size needle and I never knew that! Boy, do I feel like a dummy! I've been on this board a long time, but I don't recall reading about the bigger needle for the smaller wt. thread. I'm sure it's been written, but I must have overlooked it!"...
Thread weights go in reverse order, so the 40 weight is actually thicker than the 50, so bigger thread, bigger needle.