You might try slowly pressing a key. Does it reach the platen? Or do you need to strike it normally, as during typing, for it to hit the platen under its own momentum?
The Royal 10 QX (aka Quiet Model) (1921-1923) has a special mechanism in the segment which allows the slugs to hit the platen when struck, but pressing the keys slowly doesn't allow them to reach.
Via commentary by Brian Decker, Ted Muk, and James Grooms at https://typewriterdatabase.com/1927-royal-10.16643.typewriter