Those genes are getting the best of you.
You will hate your parents for this but it is true. Genetics play a very big role on how fast or slow you gain muscles. Actually, there are two types of fibers in muscles. Type I is called the slow twitch and Type II is referred to as fast twitch. The fast twitch is twice in density when worked out compared to slow twitch muscle fibers. If your genes are in the slow twitch, well, you may have a slightly challenging muscle-building experience.
But, don’t fret! You may have the genetic disadvantage but don’t let that get you down. Just continue on with what you are doing right on your workouts and in time, your muscles are going to show.