I have an irritating problem with my '68 TR25W, if it sits for a while (week or two) the clutch sticks so I cannot put it into gear whilst the engine is running.
I expect you are not observing the time-honoured British bike tradition of
free-ing the clutch before starting? So make it part of your normal starting routine, especially if the bike has not been used for longer than a day.
Free the clutch
before you attempt to start-up, (ignition/fuel off).
First, pull in the clutch lever, and then use the kick starter
until you feel no engine resistance as you kick (which can happen quite suddenly). The clutch plates will then have been freed off.
Once the clutch plates are free, release the clutch lever, and start up in the normal way.