If you place the Abyss Lord's Skeletal Ramsters on Betsy's platform during the final wave of The Wyvern Den, and face them to target Betsy as she arrives, most of the Skeletal Ramsters will not attack her at all. None of the Abyss Lord's other defenses have this issue - Skeletal Orcs, Skeletal Archers, and the Colossus are all capable of attacking Betsy with no problem.
Reproduction Steps:
By design, Skeletal Ramsters are not supposed to hit flying enemies. In our next patch, we are updating its defense range indicator to accurately reflect this. If you wish to discuss whether it should or shouldn't hit flying enemies, please start a discussion thread on our forums.
We are marking this particular report as Fixed as there is an identical report in the Recent Reports section, but to be clear, this issue is not currently fixed; it'll be fixed in the next update.
Thanks for the report!
Not sure if this is a Map or Hero issue, so I chose the Hero category since the problem is specific to the Abyss Lord. I've never had any other character's defenses fail to attack Betsy when placed on her platform.