Okay.. so an update here, but many of you have probably already learned this.
It's not that certain creatures are immune to poisons, it's that certain creatures (including bosses) are immune to certain properties in a poison as @Panth141 said.
It appears that if a creature has immunity to any given property in a poison it renders the entire poison useless. I suppose this means that the more properties a poison has, the less likely it is to be globally useful.
-Aletheion
It's even easier then looking at the numbers. When the poison procs, it depletes one from the stack on your weapon.
When using a poison with a property that the target is immune to, no poisons get depleted from the stack during the entire fight. I'm killing these bosses and such with all light attacks, so there are loads of them with plenty of time for the poison proc if it was going to.
I did watch the numbers as well and I see that no additional damage numbers are displayed (which makes sense since no poisons got used).
-Aletheion