Had a similar problem with my bike. Took the start switch apart (not complicated) used a little steel wool (conductive) to clean all copper contacts on the slide plate and the switch base. gave it a good cleaning with contact cleaner and electrical lube. Started right up.
Also, check under the contact on the actual slide plate. It has a tiny spring that helps push it up so that it makes a connection without getting pushed back into the slide plate. Clean that spring as much as possible and maybe pry outward on the contact a little to help. That's what was wrong with mine, was that little contact on the slide plate.
Hope that helps.
child of God