What are the current system components?

Are you sure that your ground-based video Rx is tuned to the proper channel??? It sometimes happens that even when the wrong channel is used (as with Diversity systems) you may get good reception extremely close to the Rx but then the signal quickly degrades as the onboard Tx range increases. This may be what is happening.

EDIT: Also, in the diagram you have Camera Rx, with which I am not familiar. Usually the camera just plugs straight into the Tx downlink....