Eh, I'm persistent. Normally when you're standing on top of an event, you can activate it if it's priority is below you. That works both with the normal action button and your script. But if the event below you can't be activated, but the one in front of you can, then the action button works 100% of the time, but your scripts labels for some reason don't. The only reason it sometimes works is probably because I keep trying and sometimes move from on top of one event to another.
This scenario can normally only happen if the event you're standing on is the same priority as you, but with through on(you tipped me off to that in another thread). Or it can come about regardless of priority if you completely disable action button event triggers at your XY position(small changes in "start_map_event" and "check_action_event" can accomplish this)
I have looked through your script, but am not sure why it doesn't want to work, or why it works sometimes if I move around while trying. It's a bit of a mystery.