Those trannies seem to be good for around 1000 hours.
Some times an oil change to 20w50 fully synthetic oil can add another few hundred hours but will wear the unit beyond economic repair.
However before you pull the trans , try pushing the unit on the ground.
If it pushes relatively easily the the trans is shot
I use a tilting trailer.
If a mower rolls off the trailer the trans is shot, if I have to pull it down hill the trans is fine,
A duff pulley or worn belt can also cause the trans to slip under extra load so start by either testing on a hill or get under the mower & check the belt & pullys.