This happens all the time. I block in the proper direction and see my character put her axes up in order to block, but the attack lands anyway. I also notice that it happens to other people when I attack them. Honestly, it seems like it's exclusively a Zerker problem, as I haven't seen any of my attacks get through anyone else's correct guard change/block, but it's possible that it's happening to other characters.
i think it's about stats.
when someone with high block damage hits someone with very low block damage resistance there's a chance the attack goes straight through block ( i hear the block sound but it goes through anyways and i see blood spraying/dmg being done)
That's what i ended up thinking anyways. (didn't check if this happens in duel-mode without gear as i'm always in dominions.)
i could very well be wrong.