MX900 (URC remotes in general) keypad Maintenance & Repair:
1) Open the remote (twist method) and clean the circuit board contact with some contact clean, specifically the key giving you trouble. I suggest using a magnification glass to inspect them.
2) Lightly clean the touch pad contacts.
3) If two above doesn't work; repair the keypad contact membrane. Use MG Chemical's "RUBBER KEYPAD REPAIR KIT", available from Mouser.com for about $30. I have and it has worked great.

I feel confident that using the repair kit, my MX900 should last until I get tried of it. Personally, I do not wish to deal with URC's attitude.