PTO Problem (MF 590 4wd)

[aap99ma]

I've got an MF 590 tractor which has a PTO which rotates/works when I engage it when it is not connected to anything, However as soon as I connect my power harrow to the PTO and try to get the PTO to work the gear that cause the PTO to rotate will not engage. What could be the problem? I am 100% certain that there is nothing wrong with my power harrow since I've tested it on another tractor and it works just fine.