Centrifugal pumps are pretty limited to "pressure adjustment".
Diameter and RPM is pretty well determining factors in a reachable "head pressure". The impellar basically throws the liquid.
Now if this pump worked fine and started losing developed head, and assuming impellar is fine, wear ring is worn enough that internal leakage may be the biggest problem. I have worked around Sulzer pumps, and some of them have an adjustable wear ring. Packing can also cause some problem to pressure, but most of time it will create cavitation.