I couldnt attack a crypt doctor that was standing on top of two corpses with my varrion feeder. Just got the goble corpse option.

You could try holding shift to select entities maybe? Im not sure if that works, never done it for an attack?

I have had problems with this as well. If a target is standing on a corpse it's tough if not impossible to get the carrion feeder to attack instead of eat.

zoom the map for a while, you'll be able to choose properly what action you want to take