Well, this sounds like an internal problem. You should rather send it for service with the company or change the gears with all new gears. You can also just change the gear your having a problem with.
ell typically when a transmission won't hold a gear then you have some internal damage. You will probably need to rebuild it and install a new set of gears or replace then bad one.
you need to stop using your motorcycle,because you can damage the engine and after that the repair it will be very deer,so till it's not too late you need to find a mechanic.
