Paper is pretty darned slow compared to film. I recall it is like ASA 4 or something. You would need a lot of flash power to make an exposure. I bet you just don't have enough light. An f/8 flash exposure at ASA 100 would need a exposure of about f/1.8 or 1.4 on paper (if my math is correct). I'm sure your Ektar is around f6.3 wide open . . .

You could try opening the shutter on B or T and then popping your flash several times. That might work for a static subject.