Rear door locks will not lock

I have a 2003 Chevy Tahoe and my back two doors and rear door will not lock with the remote OR the electric controls on the drivers door. I can manually lock the back doors. Once locked, the remote will unlock ALL doors (including the back doors), so I know there is power to the back door locks. This problem is with BOTH back doors and the rear door. My only problem is that the back doors must be manually locked. Does anyone have a clue what my problem is? Where I should start checking?