DIAGNOSIS: The dedicated sear burner (the one in the middle at the back) likely has its tiny gas ports clogged with grease or debris. It uses a different, more intense flame pattern that's easily disrupted.
FIX: Let everything cool. Remove the sear burner grate and the main flavorizer bars to access it. The sear burner is a short, thick tube. Use a thin, stiff wire (like a straightened paperclip) to gently poke through each of the small ports along its top. Avoid bending the wire inside.
IF THAT DIDN'T WORK: Check the gas orifice (the small brass fitting where gas enters the burner) for a spider web or blockage. You may need to remove the burner to access it—just a couple of screws.
HEADS UP: Never use a drill bit attached to a power tool for this. You can change the port size and create a dangerous, oversized flame.