Looks like it's https://screeps.com/a/#!/room/shard3/W3S35.

Spawns only regenerate energy when the total amount of energy, in all spawns and extensions, is less than 300. Right now, you have exactly 300 energy in your spawns and extensions - five extensions are full with 50 energy each, an extension in the bottom left has 4 energy, and your spawn has 46. That adds up to 300, so energy regeneration stopped.