Here is a helpful solution for the common causes:

• Improperly blended dry ingredients
  SOLUTION

  Since quick breads should not be overmixed, dry ingredients should be mixed together completely to ensure proper blending of ingredients before they are combined with the wet ingredients. Use a fork or wire whisk for this step. If dry ingredients are not mixed well, there can be pockets or lumps of ingredients, (especially leavening), which can lead to bitter or off flavors. Wet ingredients should also be well mixed before combining with dry ingredients.