this is not going to be a trivial thing to troubleshoot, and if you are not familiar with how electric forklifts work an their components work together, then this may be the repair that would be less expensive and quicker to call in a qualified service tech to deal with.
Also, having a correct serial number and an e-mail address in your profile will make things easier to get you the -correct- information
That said, have you checked the at rest connections inside the traction (directional) contactors? the "at rest" connection for the reverse contact must make good connection for the unit to travel forward.