I was playing on a brute, titan weapon. I was trying to taunt and noticed my taunt target would ignore me and continue attacking dps or whatever. I asked in help chat, and a dark melee tank confirmed he'd noticed the same. Due to suggestions of being over cap, etc, I tested with my team. We got it down to just 3 mobs, and I was not fighting. Scrapper was fighting a red con tesla knight. I taunted it and then punched it. It ignored me. Definitely not a taunt cap issues. Scrapper was not taunting. No other melee on the team. Mobs seemed to target whoever hurt the most, as though ignoring class threat level completely.
Later, I was on my katana/sr tank and tested and the same results. If I was not the FIRST to damage mobs, I could not subsequently taunt them off my teammates. That animation would happen and the sound, but they ignored me completely. If I was FIRST in and taunted instantly, I could hold the agro.
Yes, I was continuing to punch and kick, this test was not just taunt and stand there waiting.
I do not have a scrapper that has taken taunt, so have not been able to test them. Some mobs that had not yet been damaged by another player were still tauntable, but if someone else had hit them, no way. Gauntlet with first strike seemed to be the only effective "taunt" I had.
TL:DR-no other taunters, not agro cap, yes I was fighting, many power sets tested, brute and tank. taunt not pulling agro