heart rot cannot be resisted, but it can be blocked and it still won't trigger on weak hits. i'd assume that if the debuffs aren't being applied, it either is being overridden by a block debuffs buff or you're attacking magic affinity champs and getting weak hits. if neither of those are applicable, you may want to contact the support team about a potential bug.

The description of the skill is "this debuff cannot be resisted", not "this debuff cannot be blocked".  From what I understand, it means that when the game normally tries to check accuracy vs resistance to determine if the debuff is placed, it simply won't do that step for this skill.  

It doesn't mean the debuff will be placed every time though.  There are other factors that could contribute to it not being placed, including the block debuffs buff, weak hits and normal immunity (e.g. with some bosses)

There  is  also  a  mastery  that  removes  debuffs  when  they  take  too  much  damage.