Zerg LvL on 28 don't forward to MAX Level 30 for Zergling Mutation Skin

Hi, i have prob. with my zerg lvl. I’m on 28, and after it lvl up i have never Exp again ago for Max Rase LvL 30. Now, i want to know what i must done to compelete this or was it a bug. Sry for my bad english i hope for help.