Diagnosis
The glide rails for the top (utensil) rack are probably clogged with hard water scale, food gunk, or a small piece of broken plastic from a utensil that's fallen into the track.
How to Fix It
Remove the top rack by lifting it up and out. Look at the two plastic glide rails on the inside top of the tub. Wipe them clean with a damp cloth. Then, inspect the wheels on the sides of the rack itself. Spin each one and look for debris wrapped around the axles or cracks in the wheels. Clean them with an old toothbrush and some warm vinegar if there's scale buildup.
If That Didn't Work
The rack might be slightly bent from overloading. Compare it to the shape of the glide rails—it should sit perfectly flat. If it's warped, you can sometimes gently bend it back by applying pressure in the opposite direction, but go slow; the plastic can snap.
Heads Up
Don't force it or yank it out. You can derail the wheels from the track, which is a pain to reassemble. Also, don't lubricate the tracks with kitchen oil or spray lubricant—it'll attract more grime. If they need lubrication, use a tiny amount of food-grade silicone spray on a cloth, then wipe the rail, not the rack wheels.