Diagnosis
Grease and carbonized debris have built up in the slide mechanism tracks and on the underside of the grill grates.
How to Fix It
Allow the grill to cool completely. Remove the grill grates and the flavorizer bars. Lift out the sliding tray assembly. Scrape all carbonized material from the four metal tracks on the grill body using a putty knife. Clean the corresponding wheels on the tray with a degreaser. Apply a thin coat of high-temperature food-safe lubricant (like Camp Chef Slide and Grill Grease) to the tracks.
If That Didn't Work
Inspect the wheels on the sliding tray. They can warp or develop flat spots from heat. Manually spin each wheel. Replace any that do not spin freely.
Heads Up
Clean the slide mechanism every 2-3 uses as part of routine maintenance. Letting grease solidify is the main cause of sticking. Never force the slide when it's stuck.