I don't know much about A10 gearboxes and from what you describe it sounds like you can't select all the gears which indicate either a worn box or incorrect adjustment. The usual suspects are worn/bent selector forks, worn cam plate, lack of shimming on layshaft & main shaft.
Suggest you invest in a repair manual
and dig deeper into the gearbox. Usually, the cause is apparent when you look closely at all the components. Try googling A10 gearbox rebuild, there are plenty of videos and other forums with advise.