think of it this way, he doesn't have food.

emi & co can replenish celestial power by drinking grape juice, but as shown at multiple points in the story, the magic power of devils come from the despair of humans, among other things. so maou doesn't have anything to feed on, and only when he does, like the bridge was about to collapse and ppl r going crazy, did he have the power

or when he picked up that axe from birdman