PhonePe — cashback not given by phonepe on 18th april 2018

I had placed an order on 18th April on PhonePe app and since there was some offer going on and I was eligible for a cashback of Rs. 1659.14. I successfully made the payment and I got a message from them saying that my wallet limit was exhausted for the month so the cashback will be credited to my wallet on the first working day of May. Since I did not get any cashback, I raised a complaint to PhonePe customer support, they say that I am not eligible for the offer since I have already availed this offer because of some earlier transaction made on PhonePe (which I have not). I asked them to share the details of the earlier transaction (of which they said I had availed the cashback but they don't even have the details of that earlier transaction.
I hope I get some support for the complaint.