Off the top of my threadbare head:
The pinch bolts must be removed, not just loosened. I can imagine that if the columns have been in for a long time they may have ledges where the trunion goes, (but I'm not sure). Mine were tight; I had to drive them out with a long rod from the top, (and knocked out the bottom fixed blanking piece in the process
). Probably not a great idea to drive on the top edge as it will likely belly out and get even more stuck. Leave it for now, go and get a drink and when you come back they will simply fall out!