Red Buff Clarity or Bug

I noticed while jungling a few times that red buff's health regen is not consisent. One game with and red buff's regen kicked once I stopped fighting monsters and champs. But a later game with and the buff's regen never seemed to kick on. I haven't had the chance to test out if it is certain champs or just weird requirements to get the regen going. Could we have a timer like for the regen?