Make sure they land on the mob. After years of tanking I have seen mobs unresponsive to taunt spam and will go after some other toon because of who knows what. Could be LOS but I dont know why that would be when I am 10 feet from the mob.
What is also really annoying is when you are taunting multiple mobs while moving and the mob will decide to go do its own thing and stop coming after you and go for another person in the team.
Also, from what I have noticed I think some mobs spawn with a small absorb shield. Taunts do 1 point of damage and what I suspect that when the taunt lands on the mob that has this absorb shield it doesn't affect the mob, thats why you sometimes have to cast multiple times or actually hit the mob.