Haven't received my approval letter for COS from H4 to F1

I have applied for COS from H4 to F1 in April which was approved in June, but I never received the approval letter. There is no change of address. I have raised 3 service request, there is no update on those requests. Nowadays I am talking directly to immigration officer, they suggested me to raise i-824 and expedite that request, I did that and expedited i824 twice but still there is no update.
I have to travel to India so I asked my college to give me a new SEVIS id so that I can travel on initial i20 as my earlier i20 went on deferred status because I was not able to provide SEVIS with the approval letter.
I am stuck in a never ending cycle and I don’t know what to be done in this case. Can anyone help me to get the process faster to get the approval letter.
Thanks in advance.

Hi @Megha

What is USCIS saying with respect to sending the letter?

They have records of it was sent but returned undelivered by USPS.
Did USCIS sent it and it was sent back by USPS?