First thing...have you ruled out the magazine as the problem? Is it damaged, is it the correct magazine for the gun? Does the problem occur with every mag or just one?
One you rule out the mag the next step is the slide catch: is it bent so that it makes contact with the top round, what happens with 1,2,3 fewer rounds.
Point is its a trial and error process, especially with an older weapon. Do you know how heavily it was used, was it properly maintained, have you examined the slide for damage, does the slide release toggle the catch, it is spring loaded and should physically engage and release the catch mechanism.
Sorry about all the questions but it is difficult at best to give a definitive answer without knowing what you have ruled out.
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