Problem with holding aggro when tanking High King

[TWoWMaxon]

Hi all, I am a long time reader but this is my first post here.
Just some gossip, I am living in Hong Kong and playing in the Taiwan version of WoW (ppl like to call it as TWoW).
Life of tankadin is hard in TWoW (as least on my server), most guilds don't treat tankadins as 25-man tanks (even for off-tank).
I am luckily to have a guild and friends that show respect to me and never treat me as a trash tank.
Although I am not the guild's MT (which is a warrior), I have higher priority to tank any boss than any other tanks.

My guild is now farming GL and ML.
In the past two weeks the MT warrior was tired of tanking, so I had become the MT.
However I had a nightmare in tanking High King Maulgar, due to threat issue.
In the first time a warlock pulled off from me when the boss was at 3%.
During the second time a hunter (with no salvation since he had to tank the shaman) got aggro at 35%, killing a bunch of ppl before I could grab the boss back.
Although we were able to kill the boss in both encounters, I got blamed for not holding aggro properly.

I think the main cause of the problem is that, I could hardly generate threat when I got stunned by the boss, so DPS can easily catch up with me.
However warriors didn't have such problem, so what's wrong with me?
I never went OOM during the fight, so mana should not be an issue.
I also poped my wings whenever the CD was up (To maximize its effectiveness, I casted Avenging Wrath only if Consecretion and Judgement was not on CD and Seal of Vengeance had stacked to 5)
I have spec-ed fully in Improved Righteous Fury and kept RF up throughout the encounter.
And I also had Judgement of Crusader on the boss.

My rotation may also have problem though, I am too overgeared in Karazhan so I usually leave out Holy Shield and hoping the boss to crush me (my healers will kill me if they know this ).
I get too used to the following rotation: Consecretion -> Judgement of Vengeance -> Seal of Vengeance -> Consectetion -> ...
Then in 25 man, the rotation becomes Holy Shield -> Consecretion -> Judgement of Vengeance -> Seal of Vengeance -> Holy Shield -> ...
I think I should switch it to Holy Shield -> Judgement of Vengeance -> Seal of Vengeance -> Consecretion -> Holy Shield -> ...

Some information about my guildies and me.
My guildies are in T4 + badge rewards + welfare epics.
While I have about 500 spell damage (buffed) and 29% dodge (buffed) + 16% parry + 10.36% miss (I have 499 defense).
Perhaps I dodge too much so Holy Shield cannot generate enough threat?

I am very disappointed and puzzled as I have no aggro problem in Gruul and Magtheridon.

Fellow tankadins, please advise if there is anything that I can improve (gear, technique).
Sorry for my poor English as it is not my native language.

[merinos]

it's not actually a threat issue... he randomly switches target < below about 40%, the usual way is to either just taunt him back, or have the 2nd and 3rd tank stand back in the raid, taunt him off and walk him back to the 1st tank.

[ulushnar]

Yeah, he seems to reset aggro at around 50%, so if your DPS has been going hell for leather, they'll outpace the lead you made in the first half.

[Snake-Aes]

His aggro is fucked up from 50% and beyond, but he's tauntable. You have plenty of time to generate threat on him before any dpser comes. by the time people switch to him, they don't really catch up till he drops.

[Seloei]

He drops agro at 50% so as soon as he hits that, you are starting from 0 again. Omen however doesn't register it and things can get a bit hairy.

If you are having problems still, ask the dps to STOP at 50% for a short time, then keep going.

[TWoWMaxon]

Thanks you guys very much for the advices.

I think I will perform much better this week.