What can I do if I am unable to get the TDS certificate (Form 16 / 16A)?

It is the duty of every person deducting tax to issue a TDS certificate. In spite of your request, if you are denied the certificate then there is a chance that the tax deducted has not been deposited by the deductor to the government account. Please inform the department (PRO or TDS section) which will then do the needful.