You have to use some force, be sure to click it down from all sides and not just the front of the mask. If it doesn’t stay down you have a defective head. Don’t force to hard you will snap the plastic.
Mine popped back out initially as well. I had to adjust the lower jawline at the hinge a bit to get it back in place. When I snapped the helmet back in after doing that it seems to stay.
Lift the helmet up off of the jawline where it should be snapped in. That jawline is attached to a double hinge right under the chin. I had to adjust it a bit to make sure it sat flush.