Is the bat using platform behavior or did you mean it's a bat in a platformer type of game?
If using platform behavior, take note of your gravity, if it doesn't float, try gravity = 0 first.
Note that platform objects will still fall even if no gravity (free fall) unless platform behavior is disabled.
You can also use other behaviors for flying, like bullet behavior (which also has a gravity parameter).
You can adjust the gravity if you want the bats to fall down to the ground (if you are supposed to hit them in your game).
Not enough info like how you coded the fly (so I can't know why it doesn't work), how your game works, etc.