About every other magazine, my semi-auto pistol's slide will not stay open after the last round is fired. What might the problem be?

There are several items to check. First, make sure the pistol in question has a slide catch lever and is supposed to hold the slide open on the last round, small frame pistols like the model 950, 21A and 3032 do not have this feature.  Second, ensure that you are using factory ammunition and the pistol is properly lubricated.  Third, make sure that you are not resting your thumb on the slide catch.  If you have another magazine, try it.  If the slide stays open on the last round repeatedly with another magazine the follower in the magazine you had trouble with should be replaced. If the slide fails to stay open with all magazines, you should send the pistol to one of our service centers for inspection to correct this condition.