AutoHotKey: AltR + AltL + u does not work (but it works with a)

Does anyone have a guess why the following AutoHotKey-Script is not working? Everything but the last 2 lines work perfectly fine but when I press AltR + AltL + u nothing happens.

!a::ä!u::ü!o::ö!s::ß<!>!a::Ä<!>!u::Ü<!>!o::Ö

I tried replacing "<!>!u::Ü" with things "RAlt & LAlt & u::Ü" but it had no result too.

btw:<!>!x works for the following letters (and does not work for all others):wrypafhklzxcvbnm