Well, there are several possible reasons for this. It may be the level difference between you and your friend. It may also be the level difference between you and the enemy. It may be a slotting choice. It may be a power use choice.
However, it is not likely to be any one of those. It's much more likely that it's because of ALL of them.
The Threat formula, roughly boiled down, is this:
Threat = Damage * AI Mod * RangeMod * AT Mod * DeBuff Mod * (TauntDurationRemaining * 1000)
Now, for the Scrapper to pull the aggro that you've built up, he needs to have twice the threat value that you have created. There's some degradation of threat over time, but I've yet to see any extensive studies or results on just how that works, so we're going to ignore it for now.
Now, as a Tanker, your AT Mod is 4, while the Scrapper's is 3. So you've got an advantage there. Your attacks also have a debuff on them (usually a Mod of 2), whereas his don't. So that's another plus for you. However, where you're probably falling short is in the Damage department. If you can only do 50 damage per second, but the Scrapper can do 200 damage per second, he's going to be getting more aggro by a factor of 4, all other things being equal.
However, Taunt is the big kahuna here. At level 50, Taunt lasts for 45 seconds base. The taunt from your aggro aura should last about 13.5 seconds, without slotting. However, against +3s, your powers are at about 65% effectiveness, if I remember the chart correctly. Thus, the taunt values are lowered to 29.25 seconds and 8.775 seconds. That should be still enough (with the *1,000 multiplier on them, to counteract the Scrapper's Taunt values.
Here's the kicker though: the Scrapper also has an aggro aura. At only one level below the enemies, his effects work at 90% effectiveness. His aggro aura has a taunt length of 12.15. That shouldn't be enough to matter against your actual Taunt power, but will override your Taunt aura.
Assuming that the damage you're doing is equal for a second, here is how the formulas will work out:
You:
Threat = 50 * AI Mod (the same for both of you, so will be dropped) * RangeMod (same as AI mod) * 4 * 2 * (8.775 * 1000) = 3,510,000
Him:
Threat = 50 * 3 * 1 * (12.15 * 1000) = 1,822,500
So, you should still have the greater threat value than him. However, this is ignoring the damage discrepancy that you're likely to have. Putting him up to 200, we'll see this:
Him:
Threat = 200 * 3 * 1 * (12.15 * 1000) = 7,290,000
That's well over twice your threat value. If he steals the aggro from you here, you'd need a value of around 14.6 million to get it back. Let's see what you can do with Taunt:
You:
Threat = 50 *4 * 2 * (29.25 * 1000) = 11,700,000
So, basically, not enough to get it back, once you've lost it to him.
This is likely what you're seeing. If you Taunt first, you shouldn't lose aggo (in the examples I've created). However, his damage output and level advantage are a large portion of why he can get and hold the aggro over you.
Edit ->
Damn, I got that all in before anyone else responded? Really?
Also, I'm sure that this is more than what you wanted to know. I was just bored and had time to explain it in depth a little bit more.
Edit 2 ->
If you can download Mid's Hero Builder, and show us what your build looks like, we can probably help you out a bit so that you can hold aggro a bit better.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
I'll just assume Aett is right and /agree
P.S. - I have a Spines/Fire Scrapper that constantly pulls agro off of Tankers if I'm not fighting right next to the Tanker. In the case of Stone Tankers, remember they are dealing with slower recharges (and movement when it comes to it).
Its your three AoE powers (Mudpots, Taunt, Soul Drain) vs. the Scrappers three AoE powers (FSC, SC, AAO).
Hello all,
I have a question for everyone. I have a level 43 Stone/Dark tank, and I play regularly with a level 45 Fiery/Shield scrapper. I have taunt (of course), all my offensive powers are slotted with level 45 taunt IO's. Despite all this, the scrapper is attracting all the aggro that I've built. I can't even pull aggro back with my taunt power. Is this due to our level difference, or perhaps the level of our enemies we are fighting? (which are level 46/47 I believe) Thanks in advance for anyone who can help answer this question for me.