Anciens Establissements Pieper Herstal .25 Auto Pistol Problem

[Greenhunter]

I recently took my old Pieper pocket pistol out to clean it and now I am unable to pull the slide back far enough to let the 2 rails on the barrel engage the 2 grooves on the frame so the gun will stay assembled.
 
What could I be doing wrong?
 
This gun re-assembles in a very similar fashion as some of the old F.I.E. .25 auto pistols that have a separate unfixed barrel (not a Titan).
2 rails at the rear of the barrel twist into 2 grooves in the frame with the slide pulled all the way rearward.
It is not happening form. It seems I can't get the slide far enough back to allow the rails and grooves to engage one another.
 
 

[Greenhunter]

Ha! I am so embarrassed!
After I posted my problem and question, I tinkered around a bit more with that little Belgian beauty and tried inserting the magazine in before mounting the slide to the frame........and it all came together slick as snot!
 
With the magazine inserted, it allowed the slide to go all the way back to engage the slide lock lever which in turn allowed the barrell rails to come back far enough to engage the grooves in the frame. With a 180 degree turn of the barrel, the whole thing fits together again like a hand in a glove!
 
I hope this information may help someone else in the future.