Can't calibrate an analogue voltmeter: Is this normal?
Could be an issue with flat vs. upright mounting.


When the needle is too far towards the center at values close to the ends of the scale, the needle of an instrument intended for an upright mounting position might lack the gravity that puts it down just enough to yield a good reading.
When the needle is too far away from the center at values close to the ends of the scale, the needle of an instrument intended for a flat mounting position might be pulled down too far by the gravity that should not act in the direction of the needle's rotation.


Usually, little symbols indicate the specified mounting position, e.g. an upside-down T means you are to use the meter upright, and a table-like symbol similar to \$\sqcap\$ means the instrument must be used in a horizontal position.

Examples may be found in this link. (Sorry German only, but you will certainly understand vertikal and horizontal.)