What Prothwata said. This generally happens when you transform at the altar. Just move out of range so the timer starts then come back. Not sure about other shrines, but the Bangkorai one sometimes the target needs to move around a bit into a different position. Like near the water seems to work best.
I can vouch for the above mentioned method, leave the alter area until you see the timer start, then return - you don't have to eat or kill anything. Had this issue twice in Bangkorai and this solved both times
What works for me and other players I've seen is to just leave the shrine area, swipe a few times at nothing, come back in, and the option to devour usually shows straight away.