So, I got the Prince of Cannock not too long ago and it wasn't really that arduous leveling him up at first, but right around when he hit LV 8 my progress became borderline glacial and it took god knows how long to get him to LV 9, and after an hour or more of grinding he still hasn't gotten to LV 10. The Hero meanwhile leveled up three times in the span it took for me to get the Prince from 8-9.

What's the reason for this?