I hope that you have applied for your Membership with your decentralized office after passing your Final Examination in Form 2. Once the records are verified by the Institute you will be awarded the Membership Number and ICAI smart identity card is issued. Please note that you are not entitled to Membership Certificate if your name is not taken into the records of ICAI, for which you ought to apply for membership to the Institute. I assume you have followed the due process. In short, entitlement to membership is not default by virtue of passing CA Final Examination.
In the event of you receiving intimation for attending the Convocation and due to paucity of time or other prior commitments you were unable to attend the event, then in such a situation the Institute dispatches the Certificate through post.
Despite this if you have not received the certificate please write to your Decentralized office. Hope the information was helpful.