Please help... I have numerous large format floor tiles that, when tapped, sound hollow as if there are voids underneath. Our builders, and their tiler, are trying to tell me this is ok. Everything I read says it should be close to 100% adhered. I'm at my whits end with this... And to make matters worse, the tiles are no longer made so if they are lifted and they break, there are no spares. Grateful for any advice... Thanks!