The first thing should be to clean the strainer. This is located on the back of the large banjo bolt that holds the hydraulic oil supply line to the lower part of the gearcase. Undo the bolt, drain the oil, withdraw bolt with strainer attached to it. Wash in kero, refit and fill with new oil. Then post again if it does not work.
Did you disconnect/reconnect the quick couplers for the loader? They are located directly under the front loader valve and are used when dismounting the loader from the tractor. Just a quick disconnect and then reconnect.The one time I had issues with the 3pt as you describe that was the culprit.
Maxilator Accumagrapple or the WR Long Grapalator