When you took it out did you mark where the rotor was pointing or line it up to #1 before removing it? It needs to go back in the same place. At the bottom of the distributor there is either a female hex or a slot. It is where the oil pump shaft slides in. That hex / slot needs to be lined up as you drop the distributor in. Did you rotate the engine at all after the distributor was out? Hopefully you didn't! If you moved the engine you will have to line it up to #1 cylinder and look down in the hole to see the orientation of the oil pump shaft and get it so that it will slip into the drive slot/hex at the bottom of the distributor as you lower it down into position. You also need to start with the rotor a little out of location (about one plug location) so that as you slide it down in you can twist the rotor to get it all the way down and engaged properly into the helical gear on the distributor shaft.