[Edi]

A MR check on secondaryeffect does not cancel damage from the primary weapon unless that weapon is also subject to MR check.

Elf Bane is one of those things that can often be useful against high protection SCs/thugs that are magic beings, but since it'äs only AP, not AN, it's possible that the damage inflicted is not enough to pierce protection and if the MR check succeeds, the secondary does nothing either.

Toss in regen, and it might well seem like nothing is happening. I've seen Elf Bane work just fine against lesser stuff for normal damage.