There are many things that can contribute to less than stellar looking grout.
After time, grout can become dirty, dingy or moldy. No matter how much you clean it, you can't seem to get that dirt and grime off the grout. Usually the only remedy to this problem is to cut out the old grout and re-grout it.
Cracks on your grout can come from improper tile installation or house settlement, especially in new homes. When the grout cracks, it allows water to seep in behind your tile where it can wreak havoc not only on your tile but on your walls as well.
Wondering why your grout is so powdery and soft? It's because the person who installed the tile did not use enough water to mix the grout. The only solution to this is to remove the problem grout.
Low grout is caused when the installer wipes too much grout out of the grout joint during installation.