I had problems with my unit also. Tested at home and it was absolutely fine. Went on a couple of shoot and the unit wouldn't communicate on both separate shoots, intermittently. Had to get a Preston unit sent over by courier to get us out of a jam; very embarrassing and bad for my image.
I don't know but it seems that on some days I can operate for long periods of time without any significant ache in my lower back but on other days, I can't really operate for more than 5 minutes! I've tried everything to ensure my vest is sitting on my shoulders and hips on every occasion. Anyone got any ideas what the problem might be?