It's common for guitars and microphones to provide a mono recording source. If you record a mono source, the audio may only be on one side: left or right. To solve this problem, click on the small arrow next to the Arm button on an audio track and select the side in which you're hearing the audio. If you are only hearing the sound on the left side, choose Left and if you are only hearing it on the right, choose Right. This will give you a centered mono recording that you can then pan left or right in your stereo mix.