The oil level is low.

Add oil in order to obtain the proper oil level. Refer to Transmission Fluid Check .

The clutch main pressure is low.

Test the pressure. Refer to Allison service manuals.

The forward clutch is slipping.

1. Rebuild the forward clutch.
2. Replace the piston seals.

Refer to Allison service manuals.

The seal rings on the front support hub are broken.

Replace the seal rings. Refer to Allison service manuals.