The technique you're using isn't common - a lot of traditional companies don't interview on skill and technique (like Google do).
Backing up your 70% of failures, we recently hired a fashion designer at our startup (we're in an interesting market!) I decided we should use the same kind of interview we would for a developer.
Everyone we interviewed was completely baffled, but it really sorted out the wheat from the chaff. At least 80% of the people we interviewed couldn't do what they said.
Thankfully though, we've ended up with someone who is really incredible. In fact, I think she has the 10x higher productivity that us developer types talk about, and her old employer is trying to hire her back.
tl;dr - looks like you've got a normal pool of applicants!