I have a +1 to minion staff, on my necromancer skills, and leveled from 19 to 20, but all the skills stopped leveling from 20 to 21 (because of the +1 staff and all skills stopped leveling on the same XP level, including non minion skills.
I took off the +1 staff, changed it with another one, and and still no progression.
I found a fix by despecializing, and specializing it again.