Ok, so in my white2 game i've heard that storm throw is always a critical hit but with my crustle it's not a critical hit %100 of the time but always a critical hit with the rest of Pokemon. Why is this?
Crustle's Ability, Shell Armor, prevents Critical Hits:
http://www.smogon.com/bw/abilities/shell_armor
It has the ability shell armor, so it prevents critical hits. Its no glitch