Problem with a 5D and flash not firing

[Gel]

Setup:

Canon 5D mark 1
Wireless trigger (Works fine)
Bowens Pulsar transmitter and 1 receiver
Bowens GM500 Monolight.

Problem, Whether set to manual or AV mode the flash fires rarely, 1 out of every 5 or 6 shutters.

I swapped the camera for a 50D and I had a flash every time.

Is there anything obvious that might be causing this?

Thanks,
Gel

[PacAce]

Try pressing down on the transmitter from above against the hotshoe. If it fires OK when you do that, the hotshoe on the 5D may be loose. If so, you can fix it by tightening the 4 screws that holds the hotshoe to the camera body.