Question: what are the causes of poor quality muffins?
Answer: Over-Stirring. Over-stirring the batter is a common cause for dry, hard muffins. ... 
Not Enough Baking Powder. If a recipe does not include enough baking powder, your muffins will have a poor texture. ... 
Too Much Egg. If a recipe includes too much egg, the muffins will come out dry and rubbery. ... 
Overcooking.