External Microphone Not Detected by 5D Mark II

While recording a documentary on my 5d2, the camera picked up the internal mic's audio regardless of the external wireless mic setup that I had plugged in. I thought there was something wrong with the mic, so I switched to a different wireless mic but ended up with the same result... Since I was on-location with a client, I opted to plug the receiver directly into my laptop as a backup solution, which worked out fine, but it was very frustrating not knowing what was wrong with my camera/ext-mic setup... Has anyone encountered something like this before?

First time doing video on 5D2, but I've recorded videos on my 7D with ext mic before with no problems. I tried switching the ext mic's live mode menu settings from automatic to manual and back again. Is there something wrong with the mic input maybe? Menu settings? Firmware issue? Mono-to-Stereo related problem? I'd really prefer not to send the camera to a shop, so please help me with your tips and suggestions.