Doing an Instrument stage check with a student in one of our 172's. Asked him to do a magnetic turn from West to North. (Were at a latitude of like 40N if this makes a difference). But doing a standard rate turn, the compass never did catch up with the aircraft. In fact we went all the way around to east, leveled off and the compass (still on West at this point) went the opposite way (through South) and finally caught up. We tried several attempts to fly get to a North heading and could not using only the magnetic compass (not time). Our other aircraft you can do it almost every time by undershooting 30 degrees or so. Whats wrong with this compass and how can we fix it?