Pumps are divided into three basic categories – centrifugal, rotary, and reciprocating. Take any of the three types and break them down further into four parts: the rotating element, the casing, the motor, and the drive. The rotating element has a shaft, sleeves, bearings and an impeller. The casing has a pump shell, wear rings, and shaft seals. The motor and drive are intuitive. Here is a basic re mediation list: 

the belts are slipping (if applicable)
the impeller is worn
the impeller is loose on the shaft or the shaft is sheared
the casing is worn
the original motor was replaced with a slower model or with a greater amount of motor slip
the requirements have changed beyond the design specifications of the system. 

Using a manual tachometer, verify if there is a speed problem. If all of the above have been investigated and a problem still exists, then you may need to increase the maximum frequency of the drive. Note that anything beyond 60 Hz will result in a shift from a constant torque to a variable torque situation. There is a low probability that the drive will detrimentally affect the intrinsic speed of the system.   _{Your Comment}