You might want to consider your opener, and whether you’re generating enough initial threat. An example opener on multiple mobs would be:
Start in Battle Stance, Charge, Cast Sweeping Strikes, if Battle Shout is not up make sure to cast that (it’s also a good AoE threat generator, provided a.) it’s buffing party members and/or their pets, and b.) they are on the mobs’ threat tables). swap to Berserker Stance and Whirlwind, then hop into Defensive Stance, and tab sunder, beginning with your primary kill target.
I put together a Classic warrior tanking guide you may find of interest.
Good luck in the dungeons!