Homeloan interest rate adjustment

Nedbank home loan department has not been adjusting the interest rate on my home loan according to the repo rate changes made in the last few months, everytime the repo rate is adjusted they send a letter sometimes reflecting the same interest rate and instalment. What's more concerning is the new rate according to these letters is only effective the 1st of the follwing month, should it not be immediate? One one month i recrived a instalment adjustment letter with an increased instalment but no reason for the increase. I have tried contacting them to resolve on numerous occasions and asked to speak to a manager but they cant answer my questions and drop my calls everytime. This is my last resort before I go to the ombudsman.