Airtel — unfair late charges on bill payment

My last date to pay the bill is 21st of the month. Last couple of months i have paid the bill through citibank bharat bill pay on 19th of the month.in both cases they reported very conveniently that they have received the amount on 22nd of the month and charged late fee of rs 100 (+gst). When i confronted with their call center they said they will waiver the late charges without providing any explanation as to why they got the money on 22nd when the payment was done on 19th. Instead they said use airtel money as that will be faster. It seems that their systems are either inefficient or malfunctioning because of which these incidences are repeating. As airtel is struggling to keep themselves afloat because of fierce competition they may be banking on gullible customers who might pay this kind of late fee charges without confronting or realizing that they are being cheated. On the same day i paid bills for other two of my connections at vodafone and tata docomo and i got instant acknowledgement that my amount is received by them. I do not what is the problem with airtel when everybody else is able to acknowledge the payment instantly.
Can anyone at airtel customer service explain this and ensure that it does not happen again?