So how can we know what is actually under our tile?
The answer is simple - we can't. We can't if we choose a professional who will not keep these crucial things in mind. Whether they use the most expensive glue or equipment, even the leading brands, if the techniques used for installing the tiles are not correct it will nearly always end in unwanted tile movement.
What is the solution to this problem?
Using the contact agglutination, we can achieve a 100% adhesion. This is called the double-sided technique where we place the glue on the tiles and on the surface too - with this method the glue is always 'fresh' between surface and tile and undeniably gives the strongest cohesion possible. This technique does require more time however this way we have the highest chance to avoid loose tiles, uneven surfaces and unwanted noises.
I personally only use the technique mentioned above and have managed to complete several jobs with amazing results in all sort of tiling in and around a house.