It does happen like that Mose, It happen's to me if i dont keep an eye on my energy when i approach level-up.

The game is designed that way. When you level up - it fills up your bar. If your bar is already full then you gain nothing. If your bar is 1/2 full, then you gain 1/2 a bar. But you don't get a full bar of energy on top of what you have.

As Melody says, there is an item you can purchase in the BW club that allows you to add it to your existing energy. But keep in mind that you have to buy the BW Club membership, buy the item - AND you have to be enrolled in the BW club for it to work. If you let your membership expire you will no longer have that perk until you renew the membership. (You don't have to buy the item again - but you have to have an active membership to use it.)

I was really frustrated once because I logged on and my club membership had just barely run out and I hadn't noticed yet that I wasn't part of the club. And I was close to leveling up and didn't worry about it because I thought I still had a membership. I lost a ton of energy. The devs might want to consider putting in a window when your membership expires like other apps do.