POST a photograph of the propane compartment.. The small valve they open when filling is the visual liquid overfill indicator valve.. When it starts spewing liquid (Which is seen as a dense white cloud) instead of Gas (Clear) the tank is full.. Many yammer about letting air out of the tank.. There is NO AIR in the tank (Assuming it has ever been filled) to let out. I've read some real funny stuff about that valve.. What I typed was fact.
Possible issues:
Tank valve not open
Regulator failed due to foreign matter (oil) in the regulator
Excess flow valve closed (Turn tank off, have a cup of electrically brewed coffee, then turn it on VERY VERY SLOWLY, if you hear gas flow, STOP, when you no longer hear gas flow CONTINUE, repeate till it's fully open).
Well, that is as far as I can go. without more information.