Mine must have been in newer condition than I thought when I bought it, apparently. When I first got mine, I had the same problem. Had to reach in through the ejection port and push the barrel forward until it kinda "clicked" ahead a bit and was somewhat loose again, whereupon it was far enough forward for me to line the pin up and get it into place. Seems there's some kind of a little "pinch point" in the frame or something there, maybe having to do with the barrel-tilt action, that causes it to sort of hang up a little there. Whatever the case, it's pretty normal.
One thing I've been in the practice of doing when re-assembling my P11 is, once I have the slide back on the frame and locked back, I swing the muzzle down with a snapping motion of my wrist and the inertia slings the barrel forward in the slide, usually right past that little pinch point of friction so that I can insert the pin.
IMPORTANT NOTE: Make sure that you DO NOT mistakenly insert the pin BEHIND the barrel lug, or AHEAD of the hole for the assembly pin. Look through the assembly pin hole while moving the barrel back and forth to see where it sits before inserting the pin. Putting it in behind the barrel and then releasing the slide causes things to bind up tight, and it can be a real bugger to un-stick it - trust me, I've done it a couple of times, myself.
