just to explain something, the thread linked by Rizin is ment to explain why you cant attack monsters.
However, there has been an update on the NPC client. now if you get it to stuck, it will be impervious in the few seconds you will attack it. (get your best weapons and kill it before he get stucks, that will/can/may annoy more than one if it gets to be! especially dlayos xP
)
the next point is that you cant lure [means you cant travel with them a long the map] monsters anymore, they will stop move at a certain point. this point im not sure if its wished or not.
anyways, my point is that, you MAY find monsters impervious, but that doesnt mean they all are. yes sometimes its completely the NPC client which is down. sometimes its simply players who tried out something */me kill them* and got them to stuck.
search them! track them! kill them!
edit: oh and if you ask how to know which case is it, just /pet summon and try /pet follow (after went a bit away)
if your pet followed you => NPC client on, your monster is off, others may not.
if it doesnt => NPC client OFF, and so all monsters are.
