Pentax K200d and Metz AF36-4

I can't seem to get my head round p-ttl. The built in flash pre-flashes and I can visibly see it. When I fit the Metz it will not do the pre-flash no matter what setting I do on the camera.
The flash is virtually new and unused, shall I take it back to the shop??
I would appreciate some advice-thanks.

regards

Chris