So let me ask you an "easy" question: for your interview questions, are you consistent in what you ask, and do you have a consistent approach to evaluating a candidate's response?

I ask this because some of your "foundational" questions are not as simple as you might think (I could talk for literally hours about XSS and the only conclusion I could give you would be "there's no sure-fire way to prevent it", for example), and others border on pop-quiz material, which is not really a great way to evaluate someone.

Another question that's important to ask: you say you're working with recruiting companies. Have you tried not doing that? Recruiting companies don't exist to find you qualified people, they exist to spam you with résumés. Recruiting companies have been known to flat-out alter résumés to make them better match the set of keywords you said you wanted. And my own experience is that recruiting companies are the source of a huge percentage of headaches in hiring. Cutting out the middlemen can and likely will drastically increase your success rate.