This is not right. If so, it's very bad on Airtel's part. Ask them. You deserve some minimum cashback atleast. As @Sai Jai bro said, upto cashbacks are always deceiving.
yesterday i raised a complaint to airtel through email. today cc call me and says that you did not rechare thru APB, so you are not eligible for CB . payment only through Airtel payment bank are eligible for CB.