If your Sync Module is unable to communicate with one of your cameras, that camera will be greyed out as seen in the image below.
If your camera is in this state, you may need to try the following troubleshooting steps to get that camera back online.
1. Power cycle your camera.
Then wait 30 seconds and try using that camera again.
2. If your camera is powered with batteries, replace them with new 1.5v Lithium AA batteries.
3. Move the camera closer to your Sync Module.
If you continue to have difficulties with your camera, please contact Technical Support here.