Could be low pressure as TxJim suggests, or could be worn 540/1000 shift linkage internally. If tractor is equipped with 540/1000 option. What pto speed are you trying to use? 1000 rpm rarely has problem, 540 can have worn parts that won't allow full engagement of gears. Sometimes installing a new snap ring to hold pto shaft in can give enough engagement to allow pto to work for a while. May need to remove rear pto housing to replace worn parts.