"Trigger once" is a handy feature that lets you do something just one time when certain conditions are met. It's great for things like tutorials.
The problem is that people often misunderstand how it works.
Basically, "Trigger once" is true only the first time all other conditions in an event are true. After that, it's false until one of the other conditions becomes false. Then, it can become true again if all conditions are true on the next check.