I'm assuming you changed the transmission oil?
There may be an air pocket that is keeping the oil from getting everywhere. From fellow member Meofuse--
"It will take some effort to get the oil passages to bleed the air out, do this by manually pushing the tractor back and forth about 8 - 10 feet with the engine off, do this until you feel the wheels lock up. After starting the engine gradually coax it back into movement until you are confident that all is well."
Rattosh51
YT6800