Mahindra Finance — penalty amount applied without my mistake

Address:

Nagpur, Maharashtra, 411002

As every month money get deducted on 5th of every month and in dec they didnt had cheques so not even informed me and didn't deducted the amount and after few days asked cheques and when i provided that so they placed a cheque after 20th of month and without informing me.
So money was not available and they started applying penalty.
When i contact customer care they said will waived off the penalty amount but nothing happened till now

Its been more than a year