You need to take the cowling off and adjust he slip clutch as per manual. They can rust and not slip properly.
This is for our Woods 121, so you eed one for your model, but it will give you and idea.
SLIP CLUTCH ADJUSTMENT
A slip clutch is designed to slip, protecting the gearbox
and driveline, should the cutter strike an obstruction.
A new slip clutch, or one that has not been used for
three months, may seize. To free the clutch, shut tractor
off, loosen the spring tension on all six bolts, start
tractor and engage tractor PTO to slip the clutch. Shut
tractor off, tighten all bolts two turns, start tractor and
again engage tractor PTO to slip the clutch. When
clutch slips freely, shut tractor off and tighten springs
evenly as necessary to prevent slippage. Do not
exceed the maximum compression of 1-3/4" at dimension
“A”.
If a clutch continues to slip with springs compressed to
the maximum setting, check friction disc (5) for excessive
wear. Discs are 1/8" thick when new. Replace after
1/32" wear. (Minimum disc thickness is 3/32".)