How do I make the barrel not to fall though this?

Hi folks, The barrel has some physics and also the ground you see on the image.

I highlighted the part on which the barrel is able to collide with but as you can see, after that small part, it starts to fall.

Why is that?

The player is able to walk on the whole surface.

https://i.imgur.com/F7iws3a.jpg[/img]