Open the remote and clean the circuit board with some contact clean, specifically the key giving you trouble. Use a magnification glass and look at the contact points making sure they are clean. Lightly clean the touch pad contacts too.

I had a simaliar problem and cured it this way. For me, humdity and dirt worked together to build up residue on the CB and membrane.

Good Luck... The remote opens up the same way most URC remotes do.