2 people hit it already.
-trans doesn't have synchros like cars do.
-roll bike forward and/or rev engine when sitting stationary and trying to shift.
-how much oil did you add, full change, etc.
The manual has a great write-up on how to do the clutch adjustment.
Read it. Do it.
I'm with the other folks, that thinks the adjustment will 'help', yet not cure it.
Its an old bike. Do your final shifting in the last few feet coming to a stop.
(FWIW: test ride some new bikes. You'll find very similar shifting characteristics.)