Laying vitrified tiles directly over old tile without the right adhesive often leads to a hollow sound, cracked grout lines, or tiles lifting within a year.
Use a high-bond, flexible (C2 S1 or better) adhesive made for overlay work. Roughen or prime the glazed old-tile surface first, and back-butter every vitrified tile fully — a thin, patchy bed is the most common cause of failure here.