I encountered this problem several times. I want to use a character accessible using the AltGr Key but for some reason it doesn't work. As if I didn't press the AltGr key.
Usually I experience this in remote Desktop but not on my local machine. I think that this is triggered when doing a combination of those keys : Alt/Ctrl/AltGr/Shift + Enter. But I tried many of those combinations, and it didn't solve the problem.
I also figured out that I could access the characters I needed while doing Ctrl+AltGr+Key instead of AltGr+Key.