I could be doing it wrong. But I have one AoE taunt. 50% after the taunt wears off they go back to their orginal targets which isnt me. Usually the DPS or healer.
1) Don't worry about tanking weak/standard mobs. It's the DPS's job to kill them first, then move to the stronger mobs that you are tanking. If it's not silver or higher, generally speaking it's not your problem. Most pulls have only two or three mobs at most that actually require tanking... leading to the next point:
2) Most classes have a 60-second CC; use it. If you're up against multiple controllable elites (anything without the boss immunity buff), you shouldn't be fighting (or tanking) any more of them at once than necessary.
As an additional point of reference, taunts in SWToR don't work as in WoW. Rather than moving you to the top of the threat list, they only force the enemy to target you for the duration of the taunt. If you are not able to generate enough threat in that time to regain the lead, you will lose aggro again.