Cho'Gath Feast reset

I played a game of Cho'Gath today and noticed that his ult wouldn't reset consistently when I killed jungle monsters. I did notice that monsters that were lower health typically did give the cd refund but those that I was just barely killing at 1050 health would usually fail to give the partial cd refund. I was killing the units as I was still gaining feast stacks, but the mana and cd was just not getting refunded.