Speed Twin 5ta Gearbox Problem

I have a problem with my 1964 5TA, when engine is running or off, if I select 1st gear and pull in clutch lever, the bike will not roll backwards; the kickstart mechanism engages and the kickstart locks progress backwards when it jams up against footrest.

I have dismantled gearbox twice now and all the kickstart mechanism and pawl all look in perfect condition. What can be causing this lock up?