Sometimes transmission problems are actually "out of spec" brass tubes, Smitty. I have experienced this firsthand when I was making LOTS of slims.
If the tube is too tight, and you press the tranny in anyhow, it can bend--or it can be difficult to turn if you have pushed it in a little too far (which can be required if the tube is a little long---like the Fr Sing size).
Most of the time the problem is easy to diagnose, but can really only be fixed by using a different tranny and the tube that fits it.
Of course this also presupposes you use tubes and trannies from the same source---which was NOT always true when I made slims. So, I felt I was responsible for my own difficulties.