you're not building items that do dps/generate threat nor are you skilling abilities; dispersion does minor aoe damage so it won't help maintain aggro and sub zero does less damage than your team; either build the new blademail items or get spectral dagger to spam it for threat (it generates 50 base + whatever damage it does + an amount based on its level)
the old ai just attacked the nearest enemy mindlessly, now it goes for either the most dangerous foe or if it can't find one the weakest/nearest

to see if you are the highest priority target for the enemy look at the number under the boss name, that's your current threat level and as a tank it's ideally red or orange (highest/2nd highest)