It may be too low. The impeller is damaged. The damage could be caused by excessive wear, erosion, corrosion or some type of physical damage. Physical damage often occurs during the assembly process when the impeller is driven on or off the shaft with a wooden block and a mallet. Many impeller designs do not have a nut cast into the impeller hub to ease removal.
Erosion occurs when solids enter the eye of the impeller.
The solids can chip off pieces of the ceramic that are passivating the impeller, causing localized corrosion. Damage can occur if the impeller to volute, or back plate clearance is too small and the shaft experiences some type of deflection. The original clearance could have diminished with thermal growth of the shaft. In an ANSI and similar design centrifugal pumps, the normal thrust towards the volute has bent the snap ring designed for bearing retention. This can allow the rotating impeller to hit the stationary volute.