I had my back wheel out during winter because the bike was on the trainer and it of course lost some air. When I put the bike together for outside riding recently, I noticed that I'm having a hard time inflating it back, again.
The nut was indeed unscrewed and pressing in with my thumb made it open but it didn't really work with the track pump, air goes mostly by, after many tries, I got it back to full pressure when I finally got the snap sound indicating the valve opens under pressure.
In the end, I was annoyed and just replaced the tube so that I get air in on the road easily if I ever have to.
When being pumped up, the tube/valve were holding air just fine, any idea what the problem could have been and how to fix that next time, like taking off the full cap and cleaning the channel with a needle or something like that?