Icy Touch:
Instant
Chills the target for 227 to 245 Frost damage and infects them with Frost Fever, a disease that deals periodic damage and reduces melee and ranged attack speed by 14% for 15 sec. Very high threat when in Frost Presence.
Death and Decay
Corrupts the ground targeted by the Death Knight, causing 62 Shadow damage every sec that targets remain in the area for 10 sec. This ability produces a high amount of threat.
These are the mechanic on live server for you to hold aggro. If you think you are doing it right then do it your way. I DND, Icy touch, PS, and Pest even if it's on single boss target. DND + IT will make the mobs stick to you like magnet. After the rotation is Boil blood + Deathstrike to increase threat + regain HP. You only need to beef up Icy touch and DnD damage because they are 7 times threat modifier in frost presence. I usually get 250+ threats on my main target when the 2nd person (usually hunter) is only about 50+ in threat after the 1st rotation.