Here you go FLL. Read this link.
http://www.propane101.com/regulatordetails.htm
This statement could be a cause:
It (diaphragm)works in conjunction with the regulator vent which allows the diaphragm to move up and down freely. If the vent is obstructed, the diaphragm will not operate properly causing a regulator lock-up or overpressure situation
Further, it was my belief, which may be incorrect, that some venting takes place during normal operation of a pressure regulated valve.