You should press "L" to bring up the skill level window. Check to see if you have a Heavy Armor skill (Warrior, Guardian, Beserker, Shadow Knights, Palidins, Inquisitors, and Templars are the classes that get Heavy Armor skill I think). You will see a number like 50/50 meaning that you have maxed your skill. Your armor skill is not something you can raise except by gaining exp and leveling.