The main causes for cracks in the grout lines or the grout to be falling out is tile is no longer adhered to the floor. Sometimes the tiles will pop up from the floor and appear to be folding ontop of eachother. This means the only thing holding it down is the grout itself. Everytime you step on the section of the tile it moves very slightly, causing the grout to give way. The second reason for cracks is the shifting in the floors foundation. To know if this is the cause, you will usually find cracked tiles along the line route. The crack will also travel a distance from one area to another. When this happens the tiles have moved apart from eachother and caused the crackes to appear. This may also cause a tile to lift itself from the foundation and become hollow as well.
The proper way to repair this is to replace the tile with a new one and have it properly set to the floor. In some cases you can even resuse the exhisting tile becuase it is able to be removed without damaging the tile. This can only be done if the tile has a hollow sound all around it when you tap on it. If the tiles are sound and just a foundation shift, you may be able to fix the crack by removing the damaged line completely and regrouting it. If the crack reappears, than the problem is the tile itself and will need to be replaced.
As always, you can ask any questions you want and we will do our best to answer them for you. http://www.thegroutpro.com/