I tried to too for a trip in September to Nashville (9/10-9/17). No luck. Though 9/10 - 9/18 works with cert, but we'd rather get back on Saturday than on Sunday. Also when I search for the flights without cert, they are much cheaper than with cert. Also looked for a trip in August to MSP (8/11-8/14) and still no luck. So far no luck with using the cert I have leftover from last year.