Senor Quino, I was able to duplicate the extraction problem in my gun two times out of about 300 shots fired while slowly cycling the lever....what is happening in my gun is that the burr/ridge on the cartidge rim from the firing pin strike can hang on the the small flat spring inside the top of the gun, this spring called the "cartridge guide".
Using the firing pin indentation on the case rim, the case can be rotated to clear the spring, where it should come out easily instead of prying it out.
The repair should be simple on such a simple gun....that spring/cartridge guide is retained by a single screw through the top of the gun. First, make sure the screw is tight. Second, if not loose, remove the screw/spring and inspect for anything stuck behind it, pushing it down further than it should go. Third, simply filing/polishing a beveled edge on the bottom forward end of the spring should allow the case rim to slide past the spring/cartridge guide. If that doesn't easily fix the problem, order a new guide.
Caution....this is a flat spring, and any scratches left on the metal might cause the spring to crack during use...polish all work, and don't scratch up the spring.