This failed because 4 and 6 are both factors of 12, so the omega is added to the 12th member by the :nth-child(4n+4) rule, then removed by the :nth-child(6n+6) rule. Solution: Reverse the order (remove, then add).

nth-fail.scss

figure{
  @include at-breakpoint($size-20){
    @include span-columns(3,12);
    &:nth-child(4n+4){
      @include omega;
    }
    &:nth-child(6n+6){
      @include remove-omega;
    }
  }
}