Issue with realtime shadows (receive shadows)

Hi.

I've been trying to add some shadows to my game, unfortunatly, one object has an issue casting realtime shadows on itself. The 3D object is a turret made on Blender. At first the roof was made with planes, so I first thought that the issue was due to the fact that theses faces are rendered only on one side, so I added a thickness to the roof but nothing changed.

Here is a picture :

I hope that somebody knows what's going on here :/

Thanks !