If I may chime in...
I own a YBR and commute 60 miles a day on it, 4 days a week. The engine and gearbox/clutch share the same oil and some people report that changing the oil sometimes fixes gearing issues. I've personally found that it slips out of gear quite rarely, but a little more so after a few thousand miles when the oil change is due. Also, if you haven't already, check your oil level. A short ride means the engine doesn't get up to temperature so things may be a bit sticky inside. You'd also get various deposits building up from condensation etc. A good 30 mile run will burn all the rubbish out nicely. Also be firm with gear changes. Pull the lever right up with your foot swiftly.
Good luck with it.