The problem is most likely that you're forgetting to set some parameter or cookie, or are setting it to something the Web server doesn't expect. Figuring out exactly what is likely to be time-consuming and tedious; essentially you're doing some minor reverse-engineering on their Web site, and that's always time-consuming.

One trick that might help is using a packet sniffer to see exactly what a working Web browser is sending and receiving, then looking to see how what you're sending differs from that. Maybe change your script to just send the same as the Web browser does, and when it works start changing one part at a time until you see what breaks.

Good luck!