Home owners ****med out of higher installments and missing installments

Since 2014, when we had the ability to pay a lumpsum on our installments, I started keep record of what amount was paid and calculated how many installments that will cover. Last year again we paid R20000 into our bond, calculated that the installments will be covered till end June 2017. Now Nedbank Homeloans keeps demanding an arrear amount and just does not assist in resolving how they calculated the arrears. Then when the intest rate dropped, we received a letter with the new installment due, to our horror our installment increased. Upon enquiry I managed to learn that aparently Nedbank took our outstanding balance, done a recalculation, giving us a new installment. The day we signed our contract, Nedbank already calculated the amount of installments due and installments were from then adjusted according to the interest rates. How can Nedbank now go and decide to recalculate our bond to fit them and **** homeowners out of their hard earned money?

1. I need assistance from a capable person to help me track where our missing installment went.

2. I need an answer how Nedbank can decide to do recalculations on our bond to get more money from homeowners..

3. Should we not get proper assistance, will we go to the ombudsman.