Powder incorporation is almost art as it is science.  From the information given, PM should not have directly caused the problem.  Possible solutions:

  1. Increase the mixer speed - that may have changed with maintenance.
  2. Increase the liquid level, so it extends above the top of the baffles, depends on the tank straight side and baffle length.
  3. Reduce the distance between the impeller and the liquid surface, this dimension has the greatest geometric effect on surface motion and vortex depth.
  4. Type of impeller - pitched-blade works best as upper impeller.
  5. Use multiple impellers - two impellers will increase surface motion and maintain mixing.

The problem is well understood, the correction is unusual, as described the process worked before.