Weak Auto Focus Assist when on Master?

I shot a wedding this past weekend and just for grins I set one of my speedlites on a stand set as slave. I set everything up and it was all firing properly (besides the awful range without using radio poppers) but my AF assist beam was really struggling to help out my AF. I tried all of my lenses with this situation on my 5D and 1D and the only time it didn't hunt was when I would temporarily set my master flash to normal (non-master) mode.

This may be a dumb question and I may be completely overlooking something, so don't be gentle with me! John

Was the AF assist beam going at all? I don't think it has lower power settings, just on and off. Different parts of it light up though, depending on the focus point(s) selected.

The AF assist was certainly functioning, just NOTICEABLY "weaker" than normal...