Maybe grip is shoved all the way into handle bar end, should have clearance to free spin.

 

Had same issue after swapping my bar setup. I would check the stuff most people mentioned, check cables seated correctly and lube cables. For me when I put the controls on the bars I tightened down the master cylinder and front brake lever before the switch housing and the throttle tube / switch housing needed to be push on like another 1/8th or 1/16th of an inch. When that was tightened it pushed itself on and it was causing the throttle tube to rub the inside of the switch housing causing it to stick or slowly snap back. I just loosened up the switch housing and clamp for brake lever / MC, made sure everything was pushed on the bar all the way then tightened the switch housing first and then the brake lever and everything was back to normal behavior.

 

Before trying the more complicated stuff, back the throttle lock screw way out (more than you think would be necessary) and see if the condition improves.

 

im not going to read all the responses but ill give you my two cents.

I grease all my bike throttles with a good silicone grease. slippery as can be and never fails, drys up or wears out.

good luck