check your owner's manual to see if that machine has an adjustable PTO clutch. If so, adjust and retest. If not, sit in seat, turn ign sw to run, (Do NOT start engine), park brake engaged, turn on PTO switch.
Should hear a sharp click from PTO clutch . If not, PTO is not getting 12volts. Check for bad PTO switch or failed safety switches. If so, have battery LOAD tested by qualified shop, clean elect terminals , retest.
If you are familiar with VOM's or test lights, you can check circuitry by unplugging connector close to PTO... then check coil for correct ohm values (see mfgr's published specs) and supply circuit for correct power.
Also, do a simple check that deck belt tension is correct and that all related idler pulleys, mower deck spindles, etc., are turning freely.