I had the same problem using light bullet, super high velocity ammo in my Walther P22. A pistol quite similar to your Ruger. In fact it will not strip a new round from the magazine at all with Aguila SuperMax 30 grain rounds. My assumption is that the light bullet does not provide enough recoil energy to fully cycle the slide. Only enough to eject the fired round, and cock the hammer.
I don't think you made any mistake in loading your magazine.
As mentioned by Lost Sheep, the bore should always be checked for an obstruction with any odd misfire. A squib load could cause a bullet to be pushed from the case, but remain lodged in the bore.
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