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.
https://abcnews.go.com/US/police-piece-unitedhealthcare-ceo-...
You may question his methods, but you can't question his effectiveness.