The first thing I check when I see this problem is the type of oil used.
It must be MTO (Multi-purpose Tractor Oil). This is similar to Hy-guard or Hytran oils. Never use automotive fluids (like ATF).
Second is the oil level. It must be checked with the engine running AND the oil must be HOT. (160-190 degrees F)
Check the level cold will produce the wrong level.
Third, verify the oil is not overheating. This could be from operation beyond design (heavy pushing, long driving times or using a gear too high)
This could also be from blocked oil cooler. (externally or internally).
Finally, the last item to consider is the transmission or torque convertor. Inspect filter, test pressures and stall speeds.
An abused machine can have a failed torque convertor but, I always check the above items first.
If you need more help, check these items and let me know the history of this machine. Please include the full serial number of the machine with your reply (PIN).