How to avoid objects to fall down through rigidbody platforms?

Hello! My problem is, that im making a 2D game, and im just programing the basic mechanics, but it does not want to work… The player always falling down through the platform, although it both has rigidbody and box colliders, also they are at the same layers. Only the Player uses gravity, the platform not obviously

Are they on the same Z-axis?

This may sound silly but are they using Collider2D and Rigidbody2D?