Hi and welcome to the forum.
First of all, do you have the external pump driven off the front pulley or the standard internal type ?
If it's the external type, the drive couplings wear out.
If internal, it is driven off the same drive train as the PTO so make sure you have the PTO selector lever in the correct place. This is the L/H of the 2 levers between your feet. It has 4 positions. The front is Hyds only, the 2nd is Hyds and PTO, the next position is neutral and the rearmost is PTO only. also the 540/100 lever must be engaged.
If you're sure it is driving okay and the PTO clutch isn't pulled up (no light showing on the dash) the next thing to try is the suction filter which is on the aluminium housing screwed into the
hydraulic pump plate in the bottom of the rear axle. You don't need to drain the
oil to clean it.
Try that lot and let us know how you get on.
Good luck
Mike