i have a daff 2006 model prime mover . it is not engaging any gear. we have?

i have a daff 2006 prime mover with 12 speed zf as tronic gearbox . it is not engaging any gear . we have changed the gearbox computer which sits on top of the box and it has been programmed but still cant engage the gear . my dealer cant find the fault . can anyone help me and give advise what could be the problem. please advise me on . WILL BE APPRECIATED
noah