If there was some Heavy or Special Ammo drops that did not get picked up earlier in a mission, strike, etc. then for whatever reason, it will not drop anymore in later sections. It can become quite frustrating to have two weapons effectively useless simply because they were full on ammo, in the areas where the ammo for them dropped. There may also be situations where their ammo was unobtainable wherever it dropped. I suggest despawning these ammo pickups after 5-10 minutes so that they continue spawning later on.