I had one come off as well. I pushed the little white plastic sleeve onto the motor shaft as far as I could, then tightened the screw on the propellor assembly. I was able to push the propellor assembly on far enough that the white sleeve did not show. It seemed to work, the drone flies OK. I have noticed that, when a minor crash occurs, that propellor assembly seems to be the one that comes off. Since the screw simply "threads" onto the plastic sleeve, I think eventually that becomes a bit loose and comes off easier.
I would use a bit more force and try to get the plastic sleeve in as far as you can.
Hope this helps. I am a newbie myself!