gear selection

So, put my sx150 together, but have gear selection issues....
First run down the garden, all fine...BUT, couldnt get it out of gear... disconnected all cables and tried moving the gear selector at the engine end by hand..
Sometimes I can find gears, mostly not
So looks like a complete stripdown and take a look
layshaft?
Gear selector arm?
Suggestions as to where to start looking, please
Built a fair few from scratch, no issues, but this gearbox is untouched
TIA
First run down the garden, all fine...BUT, couldnt get it out of gear... disconnected all cables and tried moving the gear selector at the engine end by hand..
Sometimes I can find gears, mostly not
So looks like a complete stripdown and take a look
layshaft?
Gear selector arm?
Suggestions as to where to start looking, please
Built a fair few from scratch, no issues, but this gearbox is untouched
TIA