Just to expand on what ryclark was saying, also make sure that your mono files are going into mono tracks, not stereo ones with the same info in both channels. You can't pan stereo tracks.
Also, just as an aside, besides the pan control there's also a pan envelope you can enable--very useful if you want to move things around in real time. (I'm just thinking of that 'cause I'm working on a show that has me panning stuff all over the place mid-track.)