Have an 09 yz450f, with about 35, 40hrs on it. Have changed oil regularly approx every 5hrs. Last two times I have been out riding mx I could not shift from 3rd to 4th, this only happened once each time out felt like the shifter could not go up anymore, had to downshift then go back up. I usually use the clutch to upshift occasionally I will just let off the gas and upshift. Any ideas im thinking it could be a tweaked shift fork.