The not-so-obvious is that, perhaps, there is a build-up of deposits that's inhibiting the stem from closing. You DON'T want to go scraping or wire-brushing the stem because you can cause other problems.
The obvious is that, perhaps, the stem is turned down too tight. If you recently replaced the stem or reinstalled the old one, it may need to be loosened just a tiny bit.
At worst, replace the stems because they're all 'gummed up'.