I can't see any way the firing Pin Safety would be holding it in.
http://www.freespaces.com/laliqueinformation/Dump/pro940parts.pdf
Pushing the firing pin safety up from the bottom, interior surface of the slide seems to free up the firing pin from the firing pin safety (Part 17 on the Browning Pro9 Schematic.)
I wonder if everybody's just inserting what ever their pushing the firing pin in with, to far into the firing pin channel and are thus not able to push the hammer plate down, because the tool they are using to push the firing pin in, is actually pushing down on the inside of the firing pin channel, instead of exerting force to move the hammer plate downward?
The Hammer plate appears somewhat thinner than the one found on many pistols of this class and if the tolerances are as tight as I think they are...Just a mm or two, to far into the firing channel, would prevent you from being able to slide the plate down.
I have not tried it yet, because I don't have a tool around that I don't think would mar the plate or the pin...But thats what it looks like to me.
also be advised that FN has long history of installing parts to STAY...If you don't believe me try getting the stock tube off of an FN Tactical Police Shotgun sometime. From the factory they can be pretty tight.