Devour stacks don't proc if dragon is the first kill
I have noticed in several games that if i get devourer and first kill the dragon, i get no stacks when i should get 5. It happened to me 2-3 times before I figured it out. The devourer stacks icon is either bugged (e.g. showing some crazy number like 500 or 403) or not present UNTIL the first stack is obtained. After the first stack is obtained, it seems to work okay (although sometimes still buggy, like I kill the dragon and get no stacks even once I'm going) but for the most part it is okay.
I'm pretty sure this is happening for other people too, because I saw a kayle help her team get dragon, and she had like 2 stacks next time I saw her. I usually play Kayle, by the way.