The dipstick for the trans fluid is up front on the differential housing just behind the mast. Under the floor plate and on the top of the trans is the inching control lever arm. If the spring/springs break in there the arm can't retract all the way to allow the trans to engage. That's the most common problem that cause those symptoms.

Having a very similar problem. After a few minutes the forward or reverse movement stops working. Lifting, dropping or titlting the forks seems to reativate it sometimes for a few minutes. Strange fault.