Your Gear arnt clicking in properyl maybe due to the clutch Hold in the clutch nice and tight flick the gear make sure it 100% sounds like it is ment to be in now realse the clutch You may no be holding the clutch in tight enough or realseing it to early Which is causeing the gear to sit on the edge and a jolt will knock it out happens alot in Dirt Bikes due to rapid bumps and people not Changing gears properly of course normally you change them fast but just try this