Why does my camera not see the slave flash?

Playing around with my new EOS 300D to see what my old equipment can
do; flash this evening.

I have a Vivitar 283 and an old Vivitar slave (the little cylindrical
one); fitted them up on a tripod and put some tissues over the
built-in flash so it was just bright enough to trigger the slave.

Sometimes the shot doesn't show the flash (well not the main one),
sometimes it does. Why would that be? I took a series of shots at
1/60 f/5.6 (chosen by the camera on "P" setting) and 200ASA. If the
slaved 283 is above the subject to my right, or at 90deg to the
subject over to my left the shot comes out properly exposed, i.e. I
can see the effects of the slave flash. But if the 283 is closer in
to the line of shot (on a table down slightly to my left and in front
of me, out of view) then the shot is dark, with just the effect of the
built-in flash and the room lighting. Same exposure. And yes, I
checked that the slave had fired: first I could see it out of the
corner of my eye and also I could hear it re-charging.

The only thing I can think of is a synch problem, but I can't imagine
how it would happen. Light doesn't take an overly long time to get
across my living room ...
--

Henry Law Manchester, England