Depending on your billing cycle it may take up to two months cycle for the credit to be apply.
I write this on all reserve as I do not know which exact promotion you are referring to. Some promotions may have a number of month of use before credits etc.
I suggest you first refer to your billing. Cycle please note also that your cycle might be changed with this new promotion refer to your agreement. If any doubt or if you need assistance contact the billing department to Ensure your credit is coming!
Meanwhile I suggest you to pay your bill entirely unless otherwise advised by the billing department in such case where there would actually be flag as a billing error.
I hope it helps you.