Hi,
If you have this option selected, as mine, FCP will analyze and decide which channels to show. Some cameras record 8 channels of audio, but only 1 or 2 channels are recorded. If you set the stereo on the camera, the first 2 channels will be recorded, and the other 6 will be empty, and FCP won’t show them as there is nothing on those channels. If you set mono recording on the camera, FCP will show you mono channels and stereo channels if there are any, and hide the empty channels. You can check the inspector audio settings to see what you have.