First, you need enough airflow. My embouchure can be perfect, but if there isn't enough air blowing past the lips, no buzz is created. So if you feel that you're doing everything correctly, try to increase your volume of air. (This will require taking deeper, fuller, and more efficient breaths.
What does buzzing your lips mean?
Lip buzzing refers to the way in which brass players often begin their journey on their instruments, rightly or wrongly, by producing a buzz with their lips. Some chidren might call that "blowing a raspberry" but we are looking to create a controlled musical note with their lips.
What happens when you buzz your lips faster on a brass instrument?
The pitch of a brass instrument depends on the volume of air that is vibrating, as well as the speed at which the player's lips vibrate. ... By buzzing the lips faster or slower, the player can cause the air in the tube to resonate at different harmonics.