not issuing experience certificate

Hello

First, send them a letter requesting for the Experience certificate and set a time frame for the receiving of the same. If they don't reply or comply with your request you send him a legal notice requesting the same. 

You can also file a civil complaint under the section of labor laws in labour court. You can also file a civil lawsuit in courts. 

Regards,

Joy Bose

Ask if there are further queries

What is the probable reason for denying experience certificate to an ex-employee ?

Post all relevant facts for consideration and obligation. 

It is advisable to consult and engage a local prudent lawyer for appreciation of facts/ documents, professional advise and necessary proceeding.

You have not posted the material facts.