I've never had an issue with the center post not going back in.. My issues are, after I do get it back in, the grommet is loose or something because the center post just wants to sink all the way to the bottom. I HATE that. And then when you screw in a new carto or atty, you literally have to screw it all the way down for it to be semi-stable. I much prefer soldering them in once piece. That seems to have helped mine. But, look for the ones that have a small indentation on the underside of the + post. That will give you something to fill with solder and makes it WORLDS easier. Or you could use one that has a small center hole to fold the wire over and stick it down inside, then solder the bottom shut.
Another thing I do to help is once everything is soldered, I fill the inside of the connector with hot glue. I also strongly suggest sanding or using a file to remove the finish where your solder points will be. This helps as well.