The macro I'm using has 3 mounts on it:

#showtooltip
/use [nomod] Armored Brown Bear; [mod:alt] Grand Ice Mammoth; [mod:ctrl] Purple Hawkstrider

I have 4 slaves and 2 of them have no problem mounting themselves no matter which modifier is used. The other 2 are the ones I'm having trouble with. One of them, the alt key will not work but the ctrl key will. The other one is the reverse. I've tried having either of those toons as the active window when using the macro on the main and clicking the icon and that doesn't work either. So I'm not all together sure that it's a broadcast issue. It's like the key itself is not working in the instance of WoW. I've not experimented too much, I just click using the modifier for the mount I want to use then really quickly click w/out a modifier so that the toon that isn't mounting will while the others just say that another action is in progress. Everyone mounts......just wondering if anyone else had run across anything similar. And of course once they hit 40 I won't need the hawkstrider =D . Thanks for the input.