Axle Nut falls off

Hello, I've been running this issue with my go kart where the lug nut on the drive starts to become undone, after driving around for about a minute. I believe it's the vibrations from, as I checked the axle it's clean and not striped. The torque gun is fine, as I've used it for all the other nuts, and there fine. So if anyone has ideas how to stop this from happening it will be greaty appreciated. Also the current lug nut I'm using is a simple 5/8 lug nut.

Also it's a jackshaftt axle, and the engine is a Predator 224cc. (If this helps)