Sammy
I have yet to reproduce the problem of not being able to shoot other players so its unusual. I can only assume maybe its a certain combination of things causing it.
If you know, when it happens are there any perks/weapons that always part of your loadout? The player that couldnt be shot, did he recently use some kind of streak weapon? A few of them do set a spawn protection flag so the players cant be damaged while in them (ac130, Predator etc). But when finished the flags should be shut off.
That happens even when the map is just starting. But i found it happens more often in close combat when people are bunny hopping than when we're in a long distance. If the ennemy have no move, the kill is almost good but when he moves just a little bit I know i'll use my entiere charger to be sure cause half of bullets do nothing.