Supply Packs?

Can someone give me a concrete idea of how supply packs work? Sometimes I can fill up my ammo & explosives on them and sometimes I get nothing from them. I thought it was maybe only a squad member could resupply, but we tried that and I ended up not getting anything from it. Like I said, at other times I can stockpile rockets & mines, but other times I pick up nothing, even when standing right on top of a supply pack. Freshly dropped OR placed earlier. Seems like it might be a bug.