Damage Potions don't seem to hit always when trown onto an entity

If you spam damage pots it is still occuring. Although this is only possible in Creative, since you need more than 9 damage pots to have it occuring.
But sometimes it is not damaging, that is the truth.
(Tested in 1.8.1 & 1.8.2pre1)

Mobs have a brief moment of invincibility after being hit. The invincibility is indicated by them turning red. Of course if you spam it, they will get hit less times. This also applies to players. It is to prevent the damage spam crutch to make the game playable. If it were not for this, a mob or player could kill you in mere seconds, and vice-versa. This is a feature.

That's true. Thanks for the explanation.