If the mower has bevel boxes on the blade spindles like say Walkers do then the PTO clutch is slipping , assuming the engine speed does not reduce
If the mower has belts driving the bade spindles then the belt is slipping or one of the pulleys is stripped out .
Unfortunately Kubots IPL's are near impossible to access down here without the full model & serial numbers
We have a couple of F2400's we are slowly coming to grips with and finding information on them has been an uphill battle .
They have a shaft drive PTO to a bevel box then belts to the blades
In this case the pulley on the bevel box had sheared the drive key so when not cutting the blades spin up fine but when attempting to mow 2' tall tussock grass the best it could do was a loose plat