Out of curiosity, how old is your MX-500? I know there
was an issue with *original* MX-500s with learning certain
long codes on the LCD buttons (they supported fewer bits
than the non-LCD buttons). In the end URC removed the
ability to reset the remote to the factory default configuration,
and used that memory for the LCD keys.

After a search I found another report of this issue (with
the same keys in fact) but another poster said they were
able to do it by changing their learning technique.

So I suggest trying out some of the tricks here: [Link: remotecentral.com]