Mine do that once in a while, try pushing the poppet down with a screw driver or something and releasing it. Basically reset the seal, works most of the time for me, until I can rebuild the keg with new seals.
Don't take any more faith in anything I say than you would anyone else on the internet. If you listen to what I say, then hurt your self or break something it is your own fault, I am just expressing my opinion or experience.
THINK for your self!!