Hero does not always level up … put 5 in and Hero kevels up maybe 1 or none ?
Hero levels go up based on exp. As the levels get higher and higher, it takes more cards/exp. to go up the next one.
(This is generally true in any type of game with levels.)
Edit: For example, these two cards make very little dent at lvl 62. (See below). But the exact same two cards will raise a lvl 1 4* nearly two levels. It gets harder as you raise levels: