A friend of mine has this gun and this problem.

The firing pin and spring have been replaced.  I suggested that it might be a magazine issue.

Friend bought a new mag and the same problem exist, but now the slide is stuck to the back.

Any ideas on this problem?

Thanks

Does he have the firing pin spring guide?

I've seen two Vest Pockets and one Baby that wouldn't run because this part was lost.

Make sure the barrel has not turned slightly, it would cause the slide to stay back.

I have two and never had any issues with either one of them. I didn't run a box of ammo through them, though just a couple of rounds. It might be possible that it was assembled incorrect or the parts might not be originally FN. The slide comes back enough for you to twist the barrel, then the barrel and slide come off together. The barrels lugs is what keep it on the frame. Viewing the slide upside down, you can remove the barrel and the spring which sits between the round indicator which is visible at the rear of the slide and the firing pin. Something might be backwards or missing.
 

I messed with the gun today.  The new magazine seems to work.  The slide was stuck back because the firing pin was being held back by the sear.

I'll know more once my friend has a chance to shoot it.

Thanks for the help.