Mobile Phone Bill Rate Discounts Not Honored by Xfinity

I switched to the mobile service in August with my wife we each have a line. The monthly charges I agreed to (written in my contract) included 2 lines at about $30 per line per month. There was also a promotion that included one line free with the purchase of a line for Xfinity internet customers, so our total bill should be $30 per month with this promotion running for the first year. I have been billed now 5 times without the promotion being applied, each month I get charged the full amount and the promotional discount is not honored. I have called and addressed this issue with customer service multiple times during these first few months to get the promotion applied, but each time the best I get is a single $30 credit towards the next month's bill. My call has been transferred to another department to "accelerate" the resolution, only to find out in the next billing cycle that nothing has changed, then I call back and go through the whole situation all over again. The part that I simply cannot understand is, when I call and explain the situation with my bill to the support specialist, they can see that promotion on my account and agree that it should be billing at the reduced rate I expect, but then they proceed to not fix the issue. I am getting very tired of having to constantly call and talk to someone about this matter. This is a not-so-great first impression to leave a new mobile client.