I have a problem with a low-end trumpet ($100 range, so you get what you paid for).
The valves occasionally get stuck-ish, and won't depress fully after being pressed (they stop at around ~2/3 or 3/4 way up). This is usually after oiling the valves.
What seems to fix it somewhat is unscrewing the valves, and pushing them in and out of their seats several times - it almost feels like there's some mis-fit and you have to smooth it out, since pushing in/out encounteres slight resistance. But that fixing is only temporary, and the problem reappears soon.
Is there a set of likely reasons for the issue? Specific steps I can take to troubleshoot and resolve it? (short of buying a more expensive trumpet :)