Is the yeast fresh?
Yeast has an expiration date, and after that date will not be as effective. Yeast should be stored in a cool, dry place. Yeast may have been dissolved in too hot of water or the liquid ingredients in the recipe may be too hot, causing the yeast to die. Also, check to make sure the temperature of the ingredients; if they are too cold, it could cause some of the yeast to die.
Was the dough kneaded properly?
Dough may not have been kneaded adequately. Kneading "exercises" the gluten in the bread and gives it the elasticity to hold in the air bubbles produced by the yeast. It is these air bubbles that cause the dough to expand and rise.
Was the dough given enough time to rise?
Insufficient time may have been allowed for the dough to rise correctly.
Where was the dough placed to rise?
Dough may have been placed in too hot of place for rising. The optimal temperature for the rising dough is from 70º to 90º F. At higher temperatures; the yeast will begin to die.
What type of flour was used?
If specialty grains are being used in dough, the dough will not rise as much because these grains have a lower percentage of gluten to hold in the air bubbles that cause the bread to expand. If a sourdough starter is being used, it may not be active. Sourdough starter must be fed at proper intervals and kept at the correct temperature to keep the yeast cultures alive and active.