Employer not issuing custom experience letter

Hi I am working at an IT MNC and presently working with it from 12 dec 2011. They issue proof of employment letter which states "ABC employee is working with us at XYZ post from DD MM YYYY. This letter has been issued for proof of employment with us." Most of the IT companies in India do this. Moreover this letter is automatically generated carrying a digital signature. HR people don't entertain specific query and they don't issue any custom certificate on demand, (specially not when you need it for next job) I applied for an SSC job which needed " 2 years programming experience along with processing the electronics data". I do these operations as part of my daily job but SSC denied to entertain the auto generated letter carrying digital signature. They are saying that they need custom letter carrying above words i. e. programming experience and processing electronics data. I also went to SSC along with the affidavit which was self attested mentioning that such custom letters are not issued as per my company policy. I certify that I do these duties during my daily part of work and same can be verified but they didn't even accept it. What should i do?