Silly question but do you have the clutch all the way down at the point where 4th gear is engaging. Can do 3rd to 4th very quickly and will usually slip in if you miss time it a little. Getting 1st 2nd or 3rd is harder so you instinctively take a little more care.

Do it occasionally myself.

If gearbox oil is getting a little old and you give it a hammering it would be a good thing to do anyway. The 2 times I have changed mine it shifted really smoothly for a couple of thousand km before slowly getting a little notchy again.

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Stoner & Webber 2011 World Champions