The diverter on the manual shower valve is stiff and cannot be moved to select different settings.

The diverter on the manual shower valve is stiff and cannot be moved to select different settings. Can it be repaired?

The diverter would have seized up due to lime scale. The diverter handle can be pulled off and some WD40 or similar applied to the spindle of the valve. This will make the diverter valve turn more freely. If not the diverter valve will have to be replaced.