Diagnosis
The wheel axles are likely corroded or clogged with debris. The plastic hub can also deform from heat and load.
How to Fix It
Lift the front of the cart to take weight off the wheels. Spray the axle point where it enters the wheel hub with a penetrating lubricant. Allow it to soak. Manually spin the wheel until it rotates freely. Wipe away excess lubricant.
If That Didn't Work
The axle bolt may be overtightened. Using a wrench, slightly loosen the nut on the inside of the cart frame that holds the axle. Just a quarter-turn can release binding pressure. Retighten only enough to prevent wobble.
Heads Up
Dragging the grill can damage the leg welds and the patio surface. Always engage the wheel locks when the grill is stationary to prevent accidental movement, especially on slopes.