decreasing my installment without my concerned

I have been having a credit card with African Bank since 2012, they were deducting monthly without any problem, then last year November i changed jobs i started getting paid weekly, i advised African bank and requested that they also increase my installment to R200 every week instead of R162.50, that was actioned and deduction went off as arranged till 02 Jan 2015. Then they decreased my installment to R162.50 they deducted the amount on the 9 Jan 2015 and 16 Jan 2015 then i went online and changed it to R200 they deducted the R200 for 4 weeks and decreased my installment to R50, when i first realized that they deducted R50 on the 20 Feb 2015 i went online again to change that to R200 but it did not change this time, when i saw the R50 deduction again i log a query online requested that they changed that but it was not done, they deducted the R50 again i log another query i was not responded, i log another query yesterday Maneo respond requesting that i state my query did that but she did not get back to me and no change has been done and my account is in arrears now and i refuse to be held responsible for their mistake can someone at African bank please help me.