You've asked a difficult question to pinpoint with the information given. Obviously there's something wrong with the valve. There might be debris stuck in the seat washer. There might be a small hole in the diaphragm. There might be a hairline crack in the seat itself.
I can name others reasons but that's a good start. There are so many different valves installed out there. Each having their own individual idiosyncrasies. Most are fairly easy repairs. Good luck!
If I can't fix it, it's broken!