convergence

In the last profit warning while explaining the shortcoming of the equipment the co said "Roof and floor convergence has increased stone dilution and
reduced the rate of recovery of clean coal from ROM production."
What does this mean? Initially I thought they were talking about a problem with the new $100M mining equipment that they are trying to fix but now I'm thinking if the floor & roof are getting closer this is a problem with the coal seam thickness that cant be solved. Hope I'm wrong.