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A couple of years ago, I bought two 9-round ETS extended mags for my G43. The packaging for both mags said they are 9-rounders. One of them holds 9 rounds just fine, the other only 8, no matter how hard I press down on the last round. I was mystified. Then just the other day, I held them both up to the light (they have transparent bodies) to see if I could figure out why. Lo and behold, in the one that holds only 8, the bottom round is slightly right of the centerline, and in the one that holds 9, the bottom round is slightly left of the centerline. They’re "single stack" mags, but the rounds are slightly staggered in the column. Apparently, it matters to which side of the centerline the first round in gets pushed to when you’re loading the second round.

Is it possible that something like this is also affecting your Shield mags? I suspect that single stack mags for small pistols may be more susceptible to this kind of thing.