You should try going through Travelocity instead and see what happens. There is a space to enter in a discount code there. It may yield different results.

One nuance I've found with Enterprise is that they require you to enter in a 3-character confirmation code when entering a corporate account on their website. However, if you go through one of the OTA, they do not have such a requirement.

While this is not directly related to your predicament, it does show that the OTA's/GDS's have different nuances which you can sometimes make work to your advantage.