What does the tach on your Tractor show for PTO RPM output? Not being familiar with your mower are there belts that may be slipping? Is there an idler engagement pulley that tensions a belt to spin the blade?
If it has a shearbolt, maybe the shear bolt is sheared, but there's still enough friction between the two shafts so that it looks like the blades should be spinning fast enough to cut??
With the tractor turned off but the pto engaged, attempt to move each of the blades by hand from underneath. On most mowers the blade will yield a certain distance (maybe 6" to a foot) and then come to a stop. If the blade continues to turn then remove the pulley guards and attempt to visually identify the slip point.