hi .. i used to have this problem with my bike and according to
the mechanic, there are a few causes for this:-
1. there is a mechanism that hooks onto your gear when u engage the
next gear and that mechanism is faulty. (i dunno what the name of
the mechanism is. it's some sort of hook though...) The cost of the
mechanism is only about $30 to $50 .. but workmanship will cost
about a hundred.( they have to take out your fairings, drain your
gear oil, open up the clutch case etc. ... very troublesome.)
2. try kicking harder, or adjust your clutch tension to be abit
harder.. (it's the nut just beside your clutch, or the nut at the
end of your clutch cable leading into your clutch housing.)
3. your engine oil may be a cause as TZR suggests...
just my 2 cents .. maybe someone else may like to elaborate
?