There are many reasons to make bread heavy and dense. Including... • Using a dough conditioner, old yeast, or too much yeast. • Your bread might not be 100% whole grain. Check the flour ingredient list on the nutrition label of your bread. A lot of bread that claims to be 'whole wheat' is really just made of white flour with some whole wheat flour thrown in. • Your bread might have too much sugar in it. Sugar is added to some whole-grain bread because it's known to keep baked goods moist or crisp up the crust during baking.  · This happens when sugar is cooked at high temperatures with proteins and amino acids in the dough, called the Maillard reaction. Unfortunately, sugars also keep the texture of your bread heavy. • Not having enough gluten in your dough is another reason for making dough heavy and dense. Remember, gluten makes dough elastic and stretchy to hold bubbles of carbon dioxide created by yeast or other leavening agents.   Thus, a lack of gluten will likely result in dense loaves. Weaker wheat flours like bread flour and pastry flour contain less gluten than all-purpose (AP) flour. Bread bakers usually use bread flour.  Pastry bakers often choose AP flour for its weaker protein structure, giving them more room to add enrichment like sugar and fat without sacrificing tenderness in the crumb. • Your dough might be under-proofed. A perfectly proofed loaf of bread is one where the dough has just about doubled in size, full of air bubbles.  As you can imagine, denser loaves are typically under-proofed because there aren't enough big air bubbles to give the bread lift and volume. • You didn't use an adequate amount of steam when baking. There are several ways to inject steam into your oven. All of them involve some vessel of boiling water that will create a moist environment for the dough as it bakes.