Same happened at my rental (very old duplex with baseboards) my upstairs stayed over 65, downstairs dropped down to 57 by morning. Just check for drafty windows/doors, we had to throw a towel down in front of one of the doors, we threw a sheet up to block off our stairs so that more heat stayed downstairs as opposed to up, try blocking off areas to try and help heat build up. Our downstairs went up to 62 (and climbing) within a couple minutes and the upstairs has plenty of heat. If that doesn’t work then I would maybe call the landlord and see if he can send somebody over to make sure your system is working properly