Mono channels 'cancelling' each other out when I record with an external mic

Hello, and thank you in advance...

I'm recording audio from a rode ntg2 to a canon gl2. I use an xlr to mini jack to plug it directly into the camera, and record on L/R channels. When I'm listening to the sound on headphones as I record, I can adjust the levels, tweak it, and everything sounds great. However, when I put it into final cut I get a very faint hiss, and the speech or sound of whatever I'm recording is hardly audible. However, when I delete one of the channels, it gives the mono sound that is similar to the one I heard while I was recording (it sounds a little less sharp than when I when I was recording). This result happens regardless of using the mic attenuator or not. Do you know what's going wrong when I record? Since the ntg2 is mono should I should be recording on one channel? should I be using an xlr switch?