Oh, I think I found it. Of all places, right here:
And on further inspection I see what causes it. The notches are simply not machined all the way to the rear of the cylinder. They are just short at the tail end, and thus the hammer cannot fall into the notch.
Tho' it could be that the hammer is not made right, as well.
Maybe that would be the easiest way to resolve it. File down the lower part of the hammer.