WotLK Earth shock don´t stack on bosses

[Elis96]

Hey guys...lasts 4 weeks I found out that I didn't get Earth shock on my bosses. I know it'll interrupt me from Warr Thunder Clap. Don't know what it can be? Which spells do not match with Earth Shock?

[Blind]

Any class able to tank can apply 20% reduction to attackspeed, making earthshock (99% of the time) obsolete.
Warrs have Thunderclap, DKs Frost Fever, Paladins Judgement of the Just and Druids Infected Wounds.

[skullatbash]

Hey guys...lasts 4 weeks I found out that I didn't get Earth shock on my bosses. I know it'll interrupt me from Warr Thunder Clap. Don't know what it can be? Which spells do not match with Earth Shock?

that is how it is supposed to work.

your debuff gets removed if a stronger one is applied