My employer is not issuing the experinece letter

Hi Ajay, I worked for my previous employer for almost 4 years, out of this 50% of the time I worked onsite at USA. As per company's policy, employee can not resign while Onsite (out of India). Employee needs to be in India while resigning & serve 2 months of notice period in India. In my case I resigned when I was at on site USA. Though I didn't serve notice period of 2 months, but I served the notice period of 2 weeks which was accepted by my onsite manager. My resignation was accepted & I was issued the experience letter for the duration of my US stay (2 yrs. approx.) Overall exit was very smooth, but I was not issued with experience letter of the duration for which I worked in India. I downloaded all my Salary slips (from the day of joining till resign)(India Salary slips as well as US) so that it can be used to prove that I worked for them. I also downloaded my PF Slip, Form-16, all appraisal/increament letters, offer letter. I also forwarded my resignation acceptance email to my personal email ID. In short, I have everything to prove my employment except a formal Indian experience letter. I sent emails to HR, but they are not responding at all. Please do let me know what should I do to get the experience letter from them. Thanks, Regards, Sunil