Walter Rego suggests "bend the little tab." The follower is cast pot metal and there is no "tab" to bend. If only one magazine does not lock back, that probably indicates the "shelf" on the offending magazine is worn off a bit or is out of specification. Either way, the "fix" is to put a screw in the area of the "shelf" and grind off the top half of the head and shape the remainder, crating a larger "shelf" made out of steel that will not peen off in the future. I recall a thread where someone posted pictures of the "fix." Over the decades, I have fixed several magazines that way that would not lock back even with the proper Number 2 slide stop.