You may not have this on your tractor but I once had this issue and I thought I broke my 3pt. I inadvertently engaged my draft control to full on. When I attempted to lower the 3 pt, it only went down a small amount. I later realized the error, turned off the draft control and it worked normally.
Ok guys. Weird stuff here. I adjusted the rate valve and the 3 pt will drop fast sometimes but it won't others. I still have the problem. Sometimes while running the lever engages fine and the 3 pt drops fast, other times it barely moves. It eventually gets down but it is slow. Other times I have to kill the engine to get the lever to engage.