The subject of air blockage in valves was briefly discussed on the Compressor forum, and continuing here with photo and more information on tank valves.
Many air valves (not just Scuba) are designed to flow air most efficiently in a specific direction. Flowing "backward" is sometimes necessary, as when filling a Scuba Tank. This is not usually a problem, but under certain circumstances the reverse flow can create a restriction or even complete blockage of air flow through the valve. It is generally caused by air pressure getting behind the soft seat and pushing that seat partially out of the threaded brass carrier. An old or badly worn seat may be more likely to extrude.
Some valve designs are more prone to this issue than others. In fact, almost all versions of the US Divers J-Valve seem to be more susceptible to this problem than most other valves that I have experience with. I think it is probably due to the center hole in the seat allowing air from the fill line to get behind the seat and pushing/extruding it out of the carrier if the fill pressure is applied before opening the tank valve. The age and condition of the seat (such as how tight it is held in place in the carrier) will also have something to contribute to the issue. I see this happen in well worn line valves that do not have a hole in the seat center, such as the Sherwood/Dacor line valve. Those even have an arrow cast into the valve body showing direction of air flow. If one of these is on your fill line in a backward configuration you may eventually see a blockage occur there as the worn seat can extrude from the carrier.
Prevention of this malfunction is easy enough in most cases, if you are filling your own tanks. Just be sure that when beginning a fill that you always open the tank valve before applying pressure from the fill line. (Also don't forget that your reserve lever must be in the down position to pass air into the tank!)
If someone else is filling your tanks, politely advise them to use this procedure. It is likely that that more times the seat is accidently extruded the easier it will extrude again.
This shows a normal USD J-Valve seat on the left. The other was intentionally exposed to pressure from the fill hose before opening the tank valve. As the tank valve was opened the seat extruded. I did not turn the tank valve back off, or the seat would have been pushed back into normal position. I just bled the air off and disassembled the tank valve with it still in the open position. It clearly shows the soft seat extruded into a position that will block the incoming air flow.