alootid only works on monsters where they are classed as your kill.
For example if a player is killing a super magical poring with 10,000 hp, and they have it down to like 2k hp and you attempt to ninja it.. You will not be able to auto loot the item in question from that mob since it's assigned to another player.
Basically if you kill fresh (100% HP) mobs then it should always work.. And I personally have not experienced a time when it has not worked for me.