Nikon D70 slave flash trigger problem

I have been trying to use my Vivitar 283 flash with a small, optical,
slave trigger to throw extra light onto the background sheet.
I have used this many times in the past with my film SLR and it used to
work great.
However, with the D70, the light output from the flash does not appear
in the exposure. The flash fires, as far as I can tell, at the same time
as the camera flash.
I have confirmed this by actually photographing the flash unit, which
appears dark.
I have tried shutter speeds of up to 1/8 Sec but still the same.

I have a cheap digital camera (Kodak DX3600) and the slave works OK with it.

Any idea what is going on, this has me stumped..

Andy

What are you using for a synch speed, there maybe enough delay in the
flash/slave trigger that using one of the higher speeds 1/250 or 1/500
of a second maybe too fast. Most of these systems are built for a 1/60
or 1/30 of a second synch speed. I'd agree in setting the flash to
manual. In a confined setting running manually is easy to do.

Tom