I've seen instances in which a new Positive Post won't work. I've seen a new post end up in the same spot.
Do this::
Tighten the post, and then look at the Ceramic Deck. Observe the LIP around the post screw that is touching the ceramic deck. This is what's stopping it from tightening more.
After observing, I took my post screw out; held it with my fingers, and filed that lip down a little bit by using a
metal needle file (a fingernail file MIGHT work. Don't know). Then, screwed it back in and stuck a paper clip
through the hole (to get good leverage). Next, I used needle nose wire pliers to fully tighten the post around to where I wanted the hole. Took about 10 minutes. No more problems.