Not sure what happened, but I started a new character after d/l'ing the new DLC. Recently cleared two monster zoos on level 2 and neither of them gave me something for clearing them (and I never got the pop-up showing it either). Never saw any invisible diggles in the mass (too early probably) and I've stood around in both areas enough for any invis'd to come at me (bro). Save game attached.
If you (for example) leave the floors via stairs the Monster Zoo system stops working. Did you by chance switch floors before finishing a Zoo?
Switching floors before a zoo is finished after it has been found will stop the loot. This is designed so that only people that don't run away are rewarded.
Accidentally walking over a mysterious portal is sadly identical to leaving the floor, so it cancels your zoo as well. Just FYI.
Some monsters can teleport, and will sometimes blink out of the room. When that happens, the counter will not get to zero unless you track them down and kill them. (Or, if you get lucky, they'll wander into a trap and die.) Open all the exits from the zoo and kill and gnomes or dead heads you find in adjacent rooms. Waiting around in the zoo for them to come back won't work, as they only teleport when they are aware of you.
So that's what happened when I accidentally ran up the stair I came down while moving around to avoid the poison cloud... At least i understand why no loot
I didn't leave the level after finding it - I practically never move between levels until my current one is cleared. Also didn't run into a Mysterious Portal. I walked around in ~ 2 room radius (there weren't many nearby) and there were no monsters in those rooms either. If one had teleported, it must've kept running & running. My only guess is that there's a monster trapped in a wall somewhere, again. I fired up this save and will see if it's the same for any future zoos.
Hrmm.. I had used the new Warlock's Challenge skill on some of the creatures (noticeably during the Wight cleanup at the end). I wonder if that teleport may have messed something?
Finally. Got my Bloody Reward. It was some wounded Enraged Diggle that was, for some reason, a good ways off in a closed room. I don't have any knockback skills though.. wonder how he got down there? Diggle teleport? EDIT: Aww crap. I see that I have the Dwarven Handshake skill. That must've knocked him through stuff. I did back up a little ways now that I look. Still must've been one helluva knockback.
I've found the final wight type enemy basically in the furthest corner of the map from the zoo before. Sometimes they run very very far away. I've also had them hidden in hidden compartments(behind breakable walls) halfway across the map.
