Pressure Cooker

[Elkhound]

I posted this before, but it seems to have vanished.

I'm having trouble with my pressure cooker intermittently not coming to pressure.

It isn't overfilled.  There doesn't seem to be anything wrong with either the seal or the valves.  Sometimes in two different sessions with the same ingredients in the same amounts, one time it will come to pressure, and another it won't. 

Any advice?