I was just messing around with someone’s Olivetti with the same lose carriage problem.  Much to my dismay, I couldn’t track down the owner to allow me to fiddle with it.  I believe the pinion rack is the issue.  Locate the toothed bar underneath the carriage and push it.  If it disengages super easily, you may be missing a spring.  We’ll move on from there.
If that isn’t the issue, it’s a pawl spring problem...and all hell breaks lose.  Dear god have mercy.