Comment by sergiotapia

A bit technical:

Automatic and semi-automatic weapons work the way the do because force of the round (recoil) pushes back the bolt carrier, which a spring will then push forward again. Shot is fired, bolt carrier goes back, spring pushes it forward.

Subsonic ammunition have less charge than regular ammunition, to reduce the velocity. This also means less recoil. Combined with the spring now being too stiff, the bolt carrier will simply not move back far enough to successfully chamber a new round. So you have to manually chamber a new round between each shot. One solution is to use a light / less stiff spring that is adjusted to the force of the subsonic ammo.

Same principle for when shooting blanks.

Perhaps subsonic rounds that did not have enough charge to drive the slide. It looks like there may be a suppression device on the end. My first thought was home made.

Or it could just be a single action pistol, as noted in the article. They’re not common but there are a handful of them that are relatively easy to obtain. Competitive shooters use them and it seems that Seals are sometimes issued single action pistols too.

not a gun dude, but I read that it is common when using "subsonic rounds" for quieter shots.