@dooner could be the remote itself. Disassemble it and check the rubber membrane as well as the contacts. Clean the contacts with a pencil eraser. If that does not work, I'd try a replacement remote first and if that does not work you may have issues with the IR board inside your receiver. Start with the easy things first.
@dooner uncommon for this to be an issue on the IR board/circuitry. Usually when those go it will totally go and not just partially. Maybe it's time to take it apart and take a look at it. This document will help with that Denon-AVR-disassembly.pdf