Removing VESA Plates from monitors

[Sportsman]

I have a dual monitor setup on a twin arm stand from Duronic.I have just acquired two larger monitors and want to swap them over, but I can't unscrew the Vesa plates from the back of the existing monitors.

There are 4 screws on each plate. The screws are over length and there is a black plastic spacer tube, about 1cm long surrounding the screw. When I try to unscrew them, they just rotate, same in either direction. They won't unscrew or screw up, This is the same for all 8 screws.

The screws, screw into what appears to be a metal thread set into the hole (looking at the new monitors)

Is there something I don't know about these screws? It seems strange that it should affect all of them on different makes of monitor.

Has anyone come across this before and can anyone suggest a fix, short of extreme violence?

Thanks

[Sportsman]

Right, I can only assume that the receiving nuts in the monitor are just spinning due to cheap nasty materials, badly installed. But, all 8 of them?