There is no definitive answer to this question, as the cause of a patchy beard can vary from person to person. Some possible causes could include an imbalance in testosterone levels, genetic factors, or certain medical conditions.
Genetics, age, and hormones are the most common causes of patchy beards. Genetics and the denseness of hair follicle arrangement can cause thicker or thinner beard growth.
Why Is One Side Of My Beard Thin?
One side of my beard is thinner than the other. It’s most likely due to genetics that one side of your bead is thicker than the other. Genetics and the denseness of hair follicle arrangement can cause thicker or thinner beard growth on one side. 
Why Left Side Beard Is Not Growing?
It is possible for some areas to have bald spots, but until the problem is solved, everything can be dealt with. Genetics, stress, lack of proper diet and exercise are some of the reasons why the beard isn’t growing. 
Why Is My Beard Patchy On One Side?
Genetics, age, and hormones are the most common causes of patchy beards. Most people have little control over age and genetics, however there are ways to promote patchy beard growth and grow a full beard. 
Why Is My Beard Not Even On Both Sides?
A patchy beard is usually the result of genes passed down from parents. Your beard may grow patchy due to an unbalanced diet, hormones and stress. 
Frank Harmenstein is the lead editor of the grooming website Cut Throat Straight Razor, and has over 15 years of experience in the industry. He’s personally tested and reviewed over 200 different straight razors, and has written countless articles on the subject.