Cracking. Cracks in grout joints are most obvious even to a casual observer. Such cracks may result from any of a variety of causes. Among the most common causes is shrinkage of the grout due to excess water in the mix - in other words, the product was improperly blended. The cracks occur due to the loss of the excess water in the grout through evaporation which, in turn, reduces the grout's mass.
Some cracks in grout can be attributed to the use of dry, absorbent tile that absorbs the water in the mix, stops the hydration needed for strength and reduces the mass of the grout. If you know you are dealing with dry and very absorbent tile, you can avoid this situation by dampening the joint before grouting. Use of a latex-modified grout is also helpful in resisting this type of cracking.