I've also encountered this bug.
I've been able to reliably invoke it by having the robot pick up the girl at any point. After having the girl on the robot's shoulder, the girl will not fall (i.e. "float") and she will not respond to the jump button.
*Edit: Oh nevermind about what I said above. The issue is the elevator: Stepping onto the elevator platform disables all controls except movement and switching; Stepping off the platform re-enables all controls. What I had been doing was: lift the robot first, then lift the girl, then have the robot pick the girl up (while she is still on the elevator platform) and continue from there.
However, by doing this, the game doesn't recognize that the girl ever stepped off of the elevator, so most of her controls (and falling) remain disabled. If you put the girl back onto the elevator platform and let her walk off, normal control will be restored.