Being re-interviewed is very important. Every interviewer should be re-interviewed periodically to see the other side and to calibrate the quality of your internal pool to external scores -- Why does nobody care about that? It seems equally critical if you believed in the merit of your process. Once every 50 interviews is too far apart. Once every 20 sounds about right. If they don't "pass", they should be made to re-interview again and again until they "pass", before they can interview others again. If you find all your interviewers are not available, you know you have a broken process.