That firing pin block needs to be turned/oriented just the right way to work. BTDT. Once you get it right it seems to stay there while you drive the firing pin retaining pin back in place.
Get something to push the firing pin from the back with. Push the firing pin block spring and block into the hole in the slide, push the firing pin and you'll feel it contact the block. Push up on the block till the firing pin will move completely into the slide, then let up pressure on the firing pin block and it should hold the firing pin in until you've driven the firing pin retaining pin into place.
Remember, the firing pin block has to be turned/oriented correctly to allow the firing pin to move forward when the hammer hits it.
When you say it functions 100%, do you mean it fires when you pull the trigger? Or do you mean the slide moves back and forth, the trigger moves back and forth, the hammer moves back and forth? I ask because I had an issue with a DA/SA conversion to SA only that "functioned" fine at the table but with ammo it would only go "click". On that one it was an issue with the trigger bar not moving forward quite far enough to allow it to pop back up into position to move the firing pin block lifting arm to move the firing pin block out of the way of the firing pin. The SA trigger (front of it up inside the frame) was making contact with the frame so the trigger could not move forward enough for the back of the trigger bar to pup up into position to catch/move that lifting arm.
This is a SA trigger and the circled spot is what was contacting the frame. I little metal removal (not very much at all) allowed the trigger to fully move forward. Did you replace the trigger? Might be something to check.
