Originally Posted by DMK
I'm not really seeing how a weak recoil spring could cause this.
It couldn't. I was referring to the extractor spring under the back end of the extractor. If weak, it might allow the extractor hook to bounce out of the the extractor groove rather than extracting the case correctly.
If the recoil spring were involved, the cause would be too strong a spring. For some lighter loaded modern ammo, that could be the case that the original spring is stiff. That's why I suggested heavier bullets, as they will typically give everything a harder shove backward.
Re HiBC's observation on the link: It does appear a bit battered and if it stretched too long it could theoretically fail to unlock the barrel in time, causing unlocking to be hard. But I'm not spotting battering or rounding of the locking lugs on the barrel or their recesses in the slide. They appear sharp. Also, the slide can't go back far enough to put the hammer in half-cock if the barrel hasn't unlocked. Nonetheless, you should check this out:
Put an oiled, fired case in the chamber and let the extractor hook snap closed over it. Then move the slide back by hand and see if you feel added resistance to its travel somewhere around the point where the hammer passes half-cock but before the case clears the chamber? Also put an empty case in the magazine while you do this in case of rubbing there. If you can't distinguish an increased resistance from the hammer cocking effort, cock the hammer manually and try feeling for resistance again. Try the whole exercise without the recoil spring in place to see if you feel anything you can't account for.
Finally, cocking the hammer does take effort and is part of the normal deceleration force against the slide. Take a look at the rounded bottom edge of the firing pin stop where it hits the hammer to cock it. Look for scoring. Try putting a little grease there to see if that improves operation. High pressure grease or a moly grease would be good.