According to global management consulting firm McKinsey and Co only 10% of cost-reduction programs sustain their results three years on. In particular areas such as sales, general, and administrative costs are particularly hard to shift  while manufacturing efficiencies are easier to shift.

Whether you are using Six Sigma, or Lean etc, your improvement effort will reach some limit of performance, that for which you cannot seem to improve on. The pattern will follow the tradition learning curve where the gains you make will reach a plateau.