Diagnosis
The lid is heavy, and the magnetic strip on the holder or the steel plate on the lid isn't making full contact. Often, the adhesive fails or the lid plate gets greasy.
How to Fix It
Clean both surfaces with rubbing alcohol—the magnetic strip on the side shelf and the small metal plate on the back of the lid. Any grease or oil kills the magnetic grip. Let them dry. Press the lid plate firmly against the magnet. You should feel a solid click.
If That Didn't Work
The adhesive backing the magnetic strip might have failed. You can peel it off (it's stuck on with VHB tape) and replace it with a new strong magnet strip from a hardware store, or just use a bungee cord hooked from the lid handle to the cart frame as a backup.
Heads Up
Don't rely on the magnet when the cart is being moved or in windy conditions. It's really just for stationary storage. Always double-check it's seated before walking away, or you'll get a bent lid and a heart attack from the noise.