Currently the ammo box only gives a set amount of points (21 since the update I believe) when ammo is refilled, regardless of how much ammo was refilled. Not a lot of people tend to refill ammo other than MAYBE snipers, MGs, and mortars generally. This leads to ammo boxes not being incentivized and therefore not a lot of people putting them down.
My 2 suggestions (either one or the other needs to be implemented, NOT BOTH):
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Give the amount of points (21) per magazine replenished. Some people will have ammo pouches equipped and be grabbing a ton of ammo, and on average they will be getting at least one or two kills per magazine (sometimes more). Giving points this way is pretty much equivalent of getting an assist worth of points per magazine they receive from your ammo box.
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Only give 10 points for the initial use of the ammo box. However, when using ammo they received from that box, should they get a kill, it grants a “building assist” to the owner of the ammo box. This could be especially lucrative if placed in extremely high combat areas (which is where they make an especially large difference anyways).
Both of these ideas properly incentivize players to utilize ammo boxes, keeping their team properly supplied, and the player adequately compensated for their team support.