Typically, cold weather causes the factory lube to harden and the thumb lever to stick. Or, after many years the lube solidifies. Try heating the left handlebar with a torch to melt the residue.

Or, separate the left clamshell. You'll need a can of electrical contact cleaner (Lowes, if you can't find it elsewhere) & a tube of dielectric grease. Spray the insides liberally with the electrical contact cleaner. Rinse and repeat. Lube the moving parts with dielectric grease.

If you like go a step further, then remove the two screws below. Wire Holder (X) sits above the thumb lever if you can't remember on reassembly.

Avoid removing the thumb lever's pivot screw. It's almost impossible to get back just right.

Remove Holder B from the lower clamshell. It's just friction fitted in a notch (no screws). Then pluck Item A from Holder B.

Item A has two parts. A stationary contact plate and a movable contact cube. The movable contact is what the thumb lever moves to the left and right. Pull the copper contacts out of their slots (right pic) to clean them better. Keep track of the tiny coil springs. Spray with contact cleaner and lube with dielectric grease. 20-minutes max.

 

RONW... great, wonderful. I just need help with some experience advices. I just Knew that we are able to clean & re-lube to make it easier like new instead just use wd-40 & spray. pfttt ?. as today is 70° perfect for ride. but have to switch hard. my motorcycle always have been kept inside carport but 7 years old make sense.
i am going do what you said... your post & picture that gives me an idea how to make it move so easily like new again.
thank you for your time to help me out. I appreciate it very much