The problem is simply filtering, it sounds like. It's like grade inflation: the better grades all around don't necessarily indicate smarter students overall, but now you have to select them at a higher bar.
If you have an interview that reflects the daily work and challenges of your business, having a large pool of candidates able to do that work is a good thing. The only reason to peg your difficulty to some independent institution is to offload the trouble of making that technical interview. Your business isn't weird or failing if it doesn't have some percentage of candidates failing the interview.
You assume that the test is directly correlated with aptitude and it simply isn't true. Finding the best candidate for the job is really just a matter of filtering out the best you can hire, all the activities in the interview are designed to filter. If people start gaming the system (e.g. By practicing interview questions) that doesn't make them more apt, it simply makes interviewing (and hence filtering) much harder.
