[Unclenick]

As a diagnostic step I would put a 1/4" brass rod in through the muzzle and try to start the slide back by pushing on the rod. That's where the case head will push in recoil. It should all still move freely. Next I would take a 1/2" dowel and whittle a taper on it to go into the muzzle and try to start the slide back by pushing on the barrel from the muzzle. That push occurs in recoil, too. So does a little twist to the right from rifling spin recoil. It's not huge, but add that, too. See if that makes a difference?

If so, then something about the barrel pushing back rather than being pulled back is be causing the problem. Pushing back vs. pulling slides the opposite surfaces of the locking lug and its recess in the slide against each other. Check them both for battering or peening. Check the channel in the breech face that the barrel extension (little hood at top back of barrel) slides up and down in and the barrel extension itself for contact or battering signs.

If you find no problem operating the action by either rod or barrel, then one other thing that occurs to me to do is to check that the firing pin retracts properly and isn't gummed up. Depress it with a bent paperclip or a small drift punch. If it delays in retracting or if it is dragging in the firing pin tunnel or on the slide's manual safety, it could prevent the barrel from unlocking by hanging up in the primer indentation. Examination of the primers on your fired round would likely show the indentation biased to one side if this is happening. If you haven't done it, it's not a bad idea to depress the firing pin and remove the manual safety and then the firing pin and its return spring for examination and for cleaning and lubing with a trace of light oil, as well as cleaning and lubing the manual safety and the firing pin tunnel in the slide. (Don't lose the safety detent plunger and spring.)

In all the above I am assuming something prevents the barrel from dropping down to unlock from the slide, but only under firing conditions. The slide moves back so easily in manual operation because the wadcutters don't produce a lot of pressure or recoil for running the gun, so the springs have to be light. The easy operating mechanism is correspondingly easy to jam. Simply increasing load pressure is not a good idea as it could stress the jamming parts. You want to make this work the way it was intended to, and your loads are fine for that.