Diagnosis
The valve stem is binding. This is caused by internal corrosion, grease buildup, or a misaligned knob on the D-shaft of the valve.
How to Fix It
First, remove the stiff knob by pulling it straight off the valve stem. Inspect the interior of the knob and the valve stem for debris or corrosion. Clean both with a dry cloth. Check the alignment marker on the knob (the line) and the valve. Before reattaching, ensure the valve is in the OFF position. Push the knob on firmly, aligning the marker with the OFF detent. Test the turn.
If That Didn't Work
The issue is internal to the gas valve. With the gas supply disconnected, spray a small amount of dry graphite lubricant (not oil or WD-40) into the base of the valve stem where it enters the valve body. Work the knob back and forth to distribute the lubricant.
Heads Up
If lubricating does not resolve the grinding feel, the valve may be failing internally. Contact Cuisinart for a replacement valve assembly. Do not continue to use a valve that does not turn smoothly or click securely into OFF.