The coupons tend to have strict parameters which cause the system not to quote if they are not met.

Also, sometimes certain coupons just won't work with certain CDPs. For instance, if you're using 706900 for CDP, try a different one, like one of the AAA CDPs (215 is mine, but you can use any AAA club code for same effect), plus the coupon. Turning that around, some of the coupons are for AAA and may not work without an AAA CDP.

I've gotten to the point where I pretty much only try some combo of AAA, 706900 and any possible coupons (note: I posted a link to a list of AAA coupons a while back in the Coupon Codes thread) for my date to do rate checks. If I'm initially not happy with any of that, I try to tweak pickup/dropoff times instead. That seems to get me the best results.