They get hard with age and stop sealing so well.
replace it if that's no cure than drain the tank and inspect the valve seat.
Hard acidic water can cause pitting in brass fittings. Feel the seat with your fingers and if it's all roughed up and pitted that 'll be the culprit.

I've popped 'em off and trimmed 'em back into fine shape on a lathe, others I've treated with a heavy coat of epoxy then used a flat piece of wood and some abrasive paper double sticky taped on to get a flat sealing seat Use progressively fine abrasives starting with maybe 180 or 220 then a fine abrasive like 600 or 800 at the end.