Just played a game of Rush (which we lost) and I needed more flares, so I go up to a Support player in my squad and ask for ammo. He doesn't put any down, just ignores me.
Eventually he runs off, so I follow him hoping to help myself when he resupplies himself. He didn't drop any ammo but got killed, so I picked up his kit and guess what? That's right, he wasn't carrying any ammo. Just mortar and limpets.
There should at least be some kind of indicator on screen to show that the support player isn't going to support you...