I'm trying to use a $250 E-Cert on a $261 Fare. It doesn't take it off. Not sure why.

I try to use a $150 e-cert on the $261 fare, and it works.

The $261 is before taxes (ORD/IAH - 12/30 - 1/3, 9pm departure, 840am return).

Any help? is there a better e-cert thread?