The main body doesn't interact with gravity


I’m trying to make a traning of a quadruped robot - like the example “Anymal” - with isaac gym, but with a different file URDF. The problem is that the main body doesn’t interract with the gravity (he remains fixed constantly in a single point, no matter what gravitational force I apply), while the four arms do interact with gravity (in the image below, it seems stable, but in reality, what was just described happens).
I wanna specify that I just changed the URDF file and some parameters in the “Anymal” example, but not the NN and its input and output.
Can you help me to understand the problem?

Thank you for the response