Tahm Kench's Devour is messed up [GAMEPLAY]

Sometimes in game when i use devour to pick up minions and shoot it to clear waves there are 3 things that happens: 1.) It works normally and shoots 2.) It doesn't go anywhere and leaves the minion by Tahm as if nothing happened & 3.) It shoots but it ends up leaving the minion to have 1 hp and being invulnerable for a time being