When a creature is Immune to Mind Spells, like a Stargazer Undead, the Fortitude vs. Stun 1 round doesn't work, but what's really weird is that the Undead seems to be Immune to the damage as well.
Ergo, no (acid) damage from Acid Orb to mind-immune creatures. I'm half-tempted to see if a Protection from Aligment or Clarity would block this spell's damage entirely.
Also, I wonder if it should be mind-affecting. I don't really think the acidity would affect the mind to stun a creature.
Perhaps also add that it requires a touch attack in the Spell Description. At least IG it doesn't have that.
I thought that it was a paralysis, which undead are also immune to. Shouldn't block the damage portion though that's for sure. Mind spells are usually will saves while paralysis is fort.
Perhaps you missed your ranged attack roll?
I was sure I hit the Attack Roll on the creature that made me start this post, but today I did manage to damage and Undead Giant Bear. I will test this again on the Stargazer Undead just to be sure.