Chit fund company is doing fraud

I had lifted one chit of 500000 chit from private chitfund ltd company in the sixth month where i received 300000, the monthly amount need to pay for me was 6500 and they told these monthly installment (6500) will be same for till 10 to 15 months but suddenly from the 10th month amount has been increasing huge something like this 10th m 1 th m and 13th m they are simply saying there is no competition of people in the auction that's why amount monthly amount is increasing , company people not showing any proofs of some one really lifted chit in huge amount. I sincerely request to please advise me how to proceed in this issue.