Vex V5 Controllers will not pair with Brains

I teach 2 high school robotics classes using the Vex V5 competition super kits. Today multiple controllers would not pair to brains. My students have followed all the instructions for pairing the controllers to brains to no avail. Can I get some guidance for helping my students with this problem?

I assume you followed these instructions.

Does the controller tether to the brain and show the tethered indication ?

Are both brain and controller set to VEXnet ?

What does the VEXnet radio’s LED show, solid color, flashing ?