The importance of proper sub floor level.
(the following is an article written by Anthony Palandro, NALFA certified senior installer, which appeared in FCI Magazine)
Floor Flatness may also be a problem during installations. Installing over an uneven subfloor could result in excess movement that might cause gaping and possible breakage. All subfloors require some preparation. Industry standards specify that the surface of the subfloor must be flat to within 3/16” in a 10-ft. radius. Laying a straight edge across the floor to find the high and low spots will check the flatness. All high spots need to be ground down. To fill low spots in the subfloor, I recommend using Portland-based leveling compounds approved for this application. Allow the patching compound to dry thoroughly before proceeding with the installation. Consider using a self-drying patch. Also, never use additional layers of foam underlayment to fill voids.