Diagnosis
Bent rack guides or accumulated grease and carbon buildup in the guide channels on the side walls.
How to Fix It
Remove all racks. Inspect the four support guides (tabs) on each side wall. Ensure none are bent inward. Clean the guide channels thoroughly with a degreaser and a small brush. Wipe the rack wires themselves clean of any baked-on residue.
If That Didn't Work
The racks themselves may be warped from thermal stress or improper handling (like being dropped). Visually check if the rack frame is square. A bent rack will never slide smoothly.
Heads Up
Never force a rack in or out. You can permanently bend the oven's internal guide tabs, which are difficult to straighten without removing the side panels.