DIAGNOSIS Grease and grime have worked their way into the valve shaft, or the plastic knob itself is binding on the control panel. It's a common issue after a lot of use or exposure to the elements.
FIX Start by gently pulling the stiff knob straight off the valve stem—it should come off with a firm, straight pull. Inspect the inside of the knob and the metal valve stem it sits on. Clean both with a damp cloth and a dab of dish soap to remove any visible grease. Before putting the knob back, put a tiny, tiny drop of light machine oil (like 3-in-1 oil) or food-grade lubricant on the tip of your finger and rub it onto the valve stem. Do not spray oil directly into the valve assembly. Reattach the knob and turn it back and forth a few times to work the lubrication in.
IF THAT DIDN'T WORK The valve itself might be failing internally, which is a more serious repair. If lubrication doesn't help and the knob is still grinding or impossibly stiff, you'll likely need to replace the entire valve and knob assembly. These are available as a single part from Weber.
HEADS UP Never force a knob that won't turn. You could shear the plastic internal parts of the valve, leading to a gas leak or a knob that spins freely without controlling the flame. If it doesn't turn smoothly after cleaning, it's time for a replacement part.