When you drop a Storm Totem, it “memorises” all the enemies it can see within its range at the exact point in time when it is placed. It then blasts away at them until they are all dead. However, if any new enemies come into its range some time after its initial placement, it cannot see them at all, and they are ignored; forcing the player to drop another totem earlier than he should have needed to.
Quite a killer bug for builds that buff their totems and need them in play for as long as possible.