Breeding and hatching pools normally takes 8 hours to earn you an event item, but it will sometimes get messed up if you are breeding and hatching other monsters. Other people have told me that they are experiencing this problem with breeding, and must breed with both tree/cave to get the item. The same thing happened to me during the maze. One day, I randomly started needing to breed one monster right after the 8 hour one was bred.
I'm on hatching in the current event and it's been 12 hours. I've been constantly buying eggs to hatch, but they aren't giving me the item. It was working before when I wasn't using both caves or had a lot of monsters hatching at the same time (due to the breeding even that is currently going on), so I think it has something to do with that.