After about 250 in all tradeskills the trainers have little to nothing to teach you. Instead you have to seek further recipes from drops, the AH or Factions through rep.
For instance if you gather enough rep with the Thorium Brotherhood http://wow.allakhazam.com/db/mob.html?wmob=12944 can teach you new items.
So farm rep, money or mobs with teh drops you want. If you check the menu to the left under Trade - By Trade and scroll down you will find the various Armoursmithing recipes and where to learn them.
Good luck with it, I would recommend doing the Throrium brotherhood imperial plate quests, not much effort but a lot of thorium.