Lots of players complain about the Support class's ammo box being buggy. It is, but you can work around it.

Basically the way it works is you only get one fully functining ammo box per life. If it gets destroyed by explosives or you leave it behind, you will not be able to drop another box until you die and respawn again. You can still resupply other players, but not yourself, nor can you use your weapon while resupplying others as you have to stay there holding the box!

What you may not know is you can pick up your ammo box by standing, crouching or lying close to it, centering it in your reticle and pressing G (the default key for changing kits). If you do this you will be able to redeploy the box as many times as you wish.

Sometimes you may pick up another kit that was lying invisibly near your ammo box (dead soldiers' kits are available for some time, even though both body and kit disappear from view quite rapidly). Just press G again to switch back to your kit, then move to the opposite side of the box to pick it up without the other kit interfering.

Mmm... vanilla!