It could be that the connectors don't match.
The SMA connectors come in a number of different combinations.As in male to male,male to female,female to male,female to female and such.You can have a male tread and male connection,you can have a female thread and male connector and any combination of thread/connector.
Look into the end of the connectors and see if there is a small pin poking up proud in the center or if there is just a hole for a pin to push into.
If your just getting snow you could have two connectors screwed together that both have a hole and neither has a pin in the center so the signal is not getting through.
There are adapters available to make the connection work regardless of what combination of connectors you have on the VTX and the antenna.
SMA connector - Wikipedia, the free encyclopedia