Two simple things that most likely are missing (1) not using your taunt on cooldown on your primary target; and (2) not putting your guard on the most dangerous melee DPS.
Taunts in SWTOR work differently from other games, because they are primarily threat multipliers, not aggro grabbers.
That means that when you use the taunt what it's doing is taking the threat of whoever is at the top of the mob's hate list at the moment, multiplying it up, and assigning that new higher threat level to you (placing you at the top of the hate list). This applies even if you already are at the top of the list.
Over the duration of a boss fight, using the taunt repeatedly will make your threat grow at an increasing rate, since each time it is the total hate that is multiplied, not just a momentary value. This is why you can "outrun" even DPS who are doing vastly more damage than you are; the multiplication factor simply puts your threat so far over theirs that they can't catch up, as long as you taunt properly.
Also, since the effect is a threat multiplier and not just a fixed addition of hate, you should never use a taunt to pull. If you open with a taunt, you have no prior threat on the mob to multiply, and 0 x 0 = 0; so the taunt effectively does nothing beyond forcing the mob to attack you for a few seconds. And then DPS start hitting it, and your taunt is on cooldown.
Always open with damage first, and go through at least one or two full rotations to build some hate, before using your first taunt to start multiplying. After that, for a fight that continues longer than one taunt CD, use it every time it is available to keep building hate fast enough to stay ahead of the DPS.
Similarly, putting your Guard on the melee DPS that is generating the most threat will help to keep you at the top of the hate list, by passively discounting all of their threat generation by 25%. Usually this means you should Guard the highest geared, or most competent DPS that is on the boss with you.
DPS, for their part, should be using their threat dumps on CD, for the same reason you should be using taunt. Threat dumps keep the total threat generated by the user at a flat, low level, guaranteeing that your steeply multiplying threat will outrun it and remain higher throughout the fight. Many DPS players, however, do not even know that they have a threat dump, don't know what it does, or don't know how and when to use it.
Even so, proper taunting will keep you ahead of even clueless DPS players, as long as you are doing adequate damage yourself and taunting + guarding properly.