Not moving 424D cat.

Hi . I have a tlb which stayed a long time not working so now when I want to use it it doesn't move .The engine is great works perfect but the problem is that it doesn't move the wheels don't turn around until I put all the power of the excavator then only the wheels starts to roll but the it doesn't move.i think the issues might be the gearbox or convector .so I would like to know if there is something common that cause that Your help will be much appreciated