Two things could be at work here.

1. The damage was so low the amount blocked by your zerk BA wouldnt even be 1 point.

Lets say 8 damage got past your armor and your Zerk BA was 10%, that would mean it would be blocking .8 damage, but Dgate doesn't round up or down, it just truncates so 0 damage was blocked.

2. You were hit by an attack with AP greater then your Zerk BA.