beyond the above excellent advice,, it would be helpful
in diagnosing possible causes if you describe exactly
when/how this sticking lever started..
what were you doing.. how did it feel, how did you
respond.. did anything, seem to have any effect on it..
if it started suddely, without any warning etc,
then that is also a potential diagnostic factor..
how many miles on it.. when last serviced..
has it ever happened before, even a little bit..
if it was me and given the good advices above,
id be phoning the dealer if i couldnt fix/effect it
by external maintenance, before, riding it
incl riding it into the dealer..
lastly does it make any difference
if you deliberatly increase revs
for a moment as part of downshifting..
it should need only a fairly gentle tap down..