Andy no creeps coming? glitch?

Hi andy i found this weird bug or glitch. I dont know what is called.

No creeps coming. This happen when i minize mazebert in pause coz i need to text somebody. When i return from the game i hit pause to continue but no creeps coming. I repeatedly press the pause button but still no creep. Only when i press the next wave button the creeps come out. I never press other button. Only pause button.