DIAGNOSIS Grease and debris have gotten behind the control panel and gummed up the valve stem. This is a common issue directly behind the knobs.
FIX Pull the knob straight off the valve stem. Clean the stem with a dry toothbrush to remove surface grit. Spray a small amount of food-safe lubricant (like Weber Spray Grease) onto the stem where it enters the valve body. Work the stem in and out by turning it. Wipe away excess. Reattach the knob.
IF THAT DIDN'T WORK If it's still stiff, the valve itself may be failing internally. This is rare. Contact Weber support for a replacement valve assembly.
HEADS UP Never use WD-40 or non-food-safe lubricants here. Fumes can be drawn into the grill. Use a lubricant rated for food service equipment.