The oil level is low.

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

The clutch pressure is low.

Test the pressure. Refer to Allison service manuals.

The forward clutch slips.

Rebuild the forward clutch. Refer to Allison service manuals.

The first clutch slips.

Rebuild the first clutch. Refer to Allison service manuals.

The fourth clutch (reverse) is slipping.

Rebuild the fourth clutch. Refer to Allison service manuals.