Credit to my account promised by retentions not applied.

I renewed my contract back in September and agreed a deal over the phone and after some further discussions with retentions finally agreed on a contract with a monthly credit which amounted to the same as what I had originally been offered. I was told that the first bill may be not what I would expect as the credit would take time to be applied. However I now have a second bill which does not include the credit. Could you please private message me to review as everything agreed will be in the account notes. Thanks, Chris