They solved all the practical problem with Orion that were possible without actual nuclear testing, except one - the misfire.
So think the first time you fire. The blast plate moves up, hits the springs, slows, then recoils. ending up going almost as fast out as it did in. And then the next perfectly timed blast stops the plate, sending it back up onto the springs, and so on.
Until you get a misfire. In which the blast plate sails off the end of the guiderails and flies off into intersolar space. There wasn't any way to put stops strong enough at the end of the guiderails. For every action there is ...