Leveling up?

Im currently level 4 and keep reading you need to raise your Pokemons comfort level to level up, but the exp bar feels so random. Sometimes it works, but rn I'm trying my damndest to get it to move, upgraded 2 habitats, went back, exp didnt move at all. Upgraded 4 more, went back, still hasn't moved a bit. Anyone figured out anything more specific on this?