Sorry, yeah I should have mentioned that physics/platform behaviors cannot be both enabled.
I still don't know how you are doing this? Sounds like the barrel supposed to land on something? If so I would set it to Platform object. Set platform to disabled to start.
Then like Sami424 said, set it so, when on screen enable platform behavior(which will make it fall at the speed set in the Platform properties. If it lands on something, then that's fine.
If it's falling off screen, then set the Platform behavior back to disabled, and reset it's position back to it's original X & Y coordinates, if you want the falling barrel to be a repeatable game mechanic.
As Tomycase said, you should also disable the default platform controls as well, or else the barrel will move along with the player.
Apologies for the confusion. It's really challenging to help troubleshoot various issues, without as much information as possible. For all of this to work at least small portion of the barrel would need to be within area that would then be shown on screen when the player reaches that point.