Had same thing on 7810. Deere blamed lack of grease maintenance right away, which was not the case. I pulled pins so arm could freefall without any pivot points binding do to accused lack of maintenance and still the 3 pt. would not lower. I now put 3 pt in lower position and use a chain hooked to 3 pt and then passed through a clevis on drawbar and pull 3 pt down with another tractor. Generally I have to do this if the 3 pt has not been used in over a week or two. The real problem I found out after talking to several other 7800 or 7810 owners is the internal seal in the 3 pt cylinders are fitting too tight to move unless substantial weight is on 3 pt hitch. local dealer seems to think this is not Deere's responsibility to correct even though this problem has been around since tractor was purchased new. Tractor lifts just fine , but cannot get it to lower without using this procedure.