I just ended my practice session and, hooray! The valves didn’t get stuck, not even once. I don’t know what did the trick, because I’ve made a lot of changes since yesterday’s practice session. This is what I did:
1) Following a video, I spread some oil through the holes at the bottom of the valve casings
2) I dismantled the valves and made the springs a little longer
3) I paid a lot of attention to pushing the valves with a pure vertical force
4) I was using a hairband connecting the 3rd valve tuning slide to the 1st valve tuning slide, so the 3rd slide wouldn’t slide off the trumpet while it was on its stand (I saw this hint on a youtube video). I thought that maybe the valves where getting stuck because of pressure on the tuning slides.
Can’t tell which one did the trick, but the action of the valves today was flawless.