Why do some tiles sound hollow and others not on the same floor?

Answer

Presumably because they are hollow.

The sound you hear by knocking on a tile is always some resonance frequency of the tile (just like tapping against a glass). The vibration in a fully glued tile is damped, because the tile loses its vibration energy to the underlying floor. A tile that is loose will not couple the vibration energy to the floor as easily and will therefore resonate better.