> We do the usual 5 or 6 questionsBut what metric are you determining that those 5 or 6 questions are correct?
For example, have you put together a pool of current workers that are successful at your type of work at different companies and seen how many can pass your interview test? Its pretty easy to find a recruiter that will reach out to their network and you pay 10 employed quality engineers $1,000 that do similar work to attempt the questions. If you find a significant amount of successful engineers cannot do it, then you may want to recalibrate.
I have helped several different companies fix their hiring process, purely by recalibrating their interview test questions to their specific needs. This sometimes actually means, more difficult or simply different types of questions. But very often people just do some Google searches for interviewing at FAANG and repeat those questions without tailoring them to their specific roles.
With that said, in the case of your company, it may very well be the case that the work you do requires particularly high IQ candidates, which is actually what most interview questions at the big tech companies test. In that case, you will always have a difficulty finding candidates as there are only a limited amount of high IQ people in the world. So changing your interview questions may not apply. But when that is the case, you can often save yourself quite a bit of time by asking potential hires to take an IQ online. This will cost your organization almost nothing and will likely screen out the vast majority of people that will not be able to pass those types of interview questions.