The way things are worded it sounds like they are inspected at some regular interval, but maybe not often enough, and that they have some reason to believe that it's limited to some sub-population of the fleet ("implement a recommended inspection plan for certain airplanes in the fleet"). It could be a certain production run of aircraft, or aircraft with pickle forks from a particular supplier. Probably also increased inspection of non-implicated aircraft, as well, to see if the problem is broader than they might initially suspect.