Reliving letter

Hi Members of the panel, I have worked at an IT hardware company as an online sales professional handling their accounts for Amazon , FK and other online portals. I was also single handedly managing the product and LFR ( Croma ) business for them. I resigned on the 28th of February. I had a notice period of a month. I requested them for an early reliving on the 26th of the month after. I joined a much larger firm where my earlier employer is a business account. Post joining the same, hearing about my growth, my ex employer has been trying to find faults in whatsoever I had done in my tenure with them, and have held my reliving letter out of grudge and unhappiness, just for the reason that I left them. I had certain commitments in the business which I had disclosed at the time of my resignation and had shared all details of, so that the new person who would eventually join in my place, wouldn’t face challenges in addressing. It has been almost 60 days, where I have tried reaching them , but there has been no revert whatsoever in that matter.