Has a "trigger job" been done on it? some people when they do one take too much off of the primary sear angle that makes the hammer slip to 1/2 cock when the slide is dropped. There can be other things also but usually I have found it has to do with the sear being over stoned or worn out. but there are other things too-hammer hooks crooked/uneven, not enough pressure on the sear tine of the sear spring,+ about a 1/2 doz. other possibilities.
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