Diagnosis
The sear function works by sliding the diffuser plate open so food is directly over the firepot. If it's not getting rocket-hot, the firepot isn't producing a big enough flame at that moment.
How to Fix It
Don't just slide it open mid-cook at a low temp. First, crank the main control dial to 'High' and let the grill preheat fully for 10-15 minutes. This gets a robust fire going in the pot. Then, slide the diffuser open and give it another 5 minutes for the flames to really build up directly under the grates.
If That Didn't Work
Check that the sliding mechanism is fully opening. Grease and gunk can build up in the tracks, preventing the plate from moving all the way and blocking the flame. Clean the tracks and the edges of the diffuser plate.
Heads Up
This feature is a major grease fire risk if you're cooking something fatty. Make absolutely sure your drip tray and foil are in perfect shape, and don't leave steaks unattended. It gets seriously hot when working right.