Collision detection – body_entered signal is not output when rigid body collides with rigid body

By default, a RigidBody does not send a signal if it collides with another one RigidBody, This is usually a good thing RigidBodies can often collide and trigger many signals that do not interest you and affect performance.

If you do take care of the recognition RigidBody You should stop collisions RigidBody.contact_monitor to true and RigidBody.contacts_reported to a value greater than 0,

From the documents contact_monitor:

If this is the case, the RigidBody sends out signals when it collides with another RigidBody.