Paytmmall.com - paytm not giving the cash back of repubilc day sale

dear sir/madam,
before republic day Sale paytm has given contest in which by solving a cross word puzzle i got a promo Code Which gives Rs.*** off on cart value above ****/-
In accordingly, I have placed a order worth **** by applying the above promo code for which it has shown me a cash back of Rs.*** after post shipment of items.
Order details are
Order No. **********
Ordered On ** Jan,**** *:** pm
Total Items *
Order Total *,***

But after the products are shipped, i did not receive any cash back. When I contacted them the reply given by them through e-mail is
Dear Rama,
I would like to inform you that cashback, deals and discounts offered by Paytm are applicable only by application of the valid promo code at the time of placing an order. As per our system your order ********** is not eligible for cashback as no valid promo code was applied at the time of placing the order.
Therefore, we would not be able to process your request further.
Appreciate your understanding and co-operation in this regard. Thank you for shopping with us!
Sincerely,
Alka
Paytm Mall Care

ref:_**D**dppr._****K**JJK*:ref
I would like to bring to your notice that this is the third time happening for me. I will apply some other unique promo code which is not given in the product page while placing the order, at that moment it shows the cash back and after that no cash back will come.
When i contact paytm help they simply send the above e-mail and send a mail to rate their customer support. That is why this time i have taken all the screen shots while placing the order.

Not only that, suppose without applying promo code if we go for payment, it immediately flashes a message that
"it seems no promo code is applied- Continue anyway or select Promo".
So there no chance of placing order without applying promo code as they are saying.
I am ***% guaranteed that applied promo code in the web and same thing i opened in paytm mall app that has shown a cash back of ***/- after which i made the payment using paytm wallet Balance of *** and remaining with hdfc credit card.
I have send so many e-mails with the following images as attachment. But no Reply.
I just want the justice and they should learn a lesson the "Not to cheat customers by giving promo codes for which they cannot offer cash back."
I request you people to help me out as i am mentally disturbed for repeatedly happening it to me. Please do the needful.
Thanking you,
Rama Narayan