Grizzly 125

Having some issues with a 125 grizzly, I bought it knowing it had some issues. The one that has me stumped is I replaced the drive clutch and I tighten the nut to hold the sheaves on and it backs out every time the wheeler is running. I know all the parts are correct and all there. I feel I’ve tried everything other then welding the darn thing on. ANY suggestions?
Thanks