If an automatic, is the ignition lock out switch properly adjusted? This is the brown small rectangular box on the firewall with the small lever arm.
Also check that the smaller wire going to the starter is on the proper connection. We just ran into this problem the past weekend working on getting a 280 SL back on the road. Everything was in place, but nothing from the starter. We pulled it out of the car, set it on the bench, and found that we had connected the smaller wire to the wrong post on the starter solenoid. There was an extra connection that we did not need. We changed it around and voila!