Definition of Confirmation:
This is the last and also one of the most critical steps of a design of experiments (DOE). This is the phase of setting your process at the settings you have calculated to see if you really get what your equation says you should.
This should always be done before advertising results and implementing new factor settings, confirm the results.
If you are still unable to have a confirmation, there is likely a problem with the DOE. There may be an interaction involved and/or the DOE may have been botched.