Mag Compass errors?

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?

Up here around 47 we get that all the time.

Just goes to show you what happens to people who try to fly IFR using a compass. What were you thinking anyway?

Uh, I don't know, might be low on fluid, might have been in too steep of a bank, might be someone put their headset on top of the compass between flights one too many times, might just be a fluke of where you were flying.

Also if it is a float type compass they are only good for up to 18 degrees of bank. Past that they will get stuck.

Dash was clear, bank was 15 degrees, full of fluid, other aircraft (cessna 152 and 172 works great in the same area). Wasnt just a fluke, as another instructor reported the same thing. Completely unreliable in a turn.

Sounds like its time to get a new compass, though scientists and geologist have been raving about the Earths Magnetic Poles being in the process of switching .