Thanks so much for helping me out with this. The problem appears to be the way I'm installing the 2 horizontal black brackets under the seat. I'm unable to figure out exactly how to position them so that they latch onto the bracket, which, according to the photos you posted, I've installed correctly.
It would be extremely helpful to me if you could post photos of the underside of your seat, with all hardware installed, since this is the area I'm having such difficulty with.
Thanks again, Morgan.