A certificate of competency, commission or elevator contractor license may be suspended or revoked by the director upon recommendation of the board for incompetence, neglect, misrepresentation or failure to comply with the requirements of this act, or with the rules made by the board. Such a document shall not be suspended or revoked until a hearing has been conducted after at least 15 days’ written notice has been mailed to the holder of the document by ordinary mail. The hearing shall be held before the board not more than 30 days after the date of the notice, unless the hearing date is extended for cause shown.