Did you use the site agent? They are very nice, professional and represent the builders interests and not yours. If you used an outside agent, the time to get this resolved was before you signed the agreement. Builders don't like to discount and they often have arrangements with specific lenders that may benefit them.
If you have a choice, determine the most likely time period you expect to live in the home. Compare the total cost of ownership using both lenders over that period and choose the deal that is best for you.
Before you further commit ask the builder to reconsider allowing you to have the discount but using your lender. The worst they can say is no.