You must balance everything that rotates with the shaft such as the impeller, sleeve, sleeve gasket, drive key, seal, bearings, coupling, motor etc. It never was balanced. Some of the product has attached it self to the rotating assembly. The impeller is the most logical place to look for un balance problems, especially in the balancing holes. Erosion can remove metal from the rotating parts.
Corrosion can do the same.
A non concentric sleeve, seal, impeller, coupling, etc. A piece was damaged during the installation process. The rotary unit is dragging something around with it. A piece left over from the last seal change. No one notices that one of the springs has fallen out and is resting in the bottom of the stuffing box, getting ready to be picked up by the new seal.