These valves wear out over time. Mine is almost 12 years old with 1,385,000 miles on it and it's at the end of it's life.
Mine is at the bottom end of the DOT regulations (25 PSI).
I bought a new one and will install it this coming week.
Mine was also sticking when I would release the trailer brake, a few love taps set it back in place.
Do your check again and it should pop between 40 and 25 PSI as previously stated. If it doesn't then get a new one. It was pretty cheap ($199). I'm saving the labor and installing it myself.