7810 Pto Won't Turn Under Load

New to the forum, any help appreciated.

I am trying to round bale with a John deere 7810, but the pto will not turn under load. If it is unhooked, the shaft will spin, but not when attached to the baler. Is this electrical? A hydraulic pressure issue or what?

It did it to me one day, then after flipping the switch on and off, it began working again fine for the rest of the day. Now it will not run the baler at all.

Should I troubleshoot electrical or is this more severe?

Thanks