Every 1.5 minutes you get 1 point of energy, providing you have fewer than 60 points (it is the maximum number of energy points for level 32). As soon as you reach 60 energy points, the automatic refill stops working.
If you want to add more energy to the energy bar, make it in the Cauldron or in other special buildings. There's no limit to how many energy points you can add to the energy bar.
If you're sure your energy doesn't refill on its own, even when it's under 60, let us know, and we'll check what's going on.