You have developed muscle imbalances
A muscle imbalance happens when one muscle or muscle group is stronger than its complementary/opposing muscle. This can severely limit your ability to train effectively, and may even lead to increased risk of injury in the long-term. It is very important that you recognize if you’re targeting the muscles you think you are and recognize whether you have developed a muscle imbalance which tends to alter the movement pattern.
A very common example of muscle imbalance in women is stronger quadriceps and tighter, weaker hamstrings, the result of sitting for long periods of time, wearing high heels and improper workouts. If you think you may have an imbalance, go and see a physical therapist, who will prescribe some specific movements to even the imbalance out.
Your form is bad
If you want to stay in the muscle building game for a long time, you will have to learn not only to lift the weight but also lift it in a safe and controlled manner. If you’re not doing the movement correctly, it will be impossible to make any significant progress.
For someone that is just beginning to work out, it can be very helpful to train with a personal trainer who’s had lots of experience in order to learn proper execution form. The same goes for experienced lifters, as well. If you’re not sure about how to perform a specific movement, it’s always better to ask. If you aren’t training the correct muscles, you can’t expect them to grow bigger.
Your genetics may be a detriment to your progress
It’s a fact that your genes play a huge role in regards to your potential to build muscle. Generally, there are two types of muscle fibers: type I, also known as slow-twitch fibers and type II, also known as fast-twitch fibers. Depending on which type you have more of, you might be able to either build muscle fast or slow. Fast-twitch fibers are twice as thick as slow-twitch fibers, lending to the thickness of the muscle when it’s inactive.
The people who have a genetic predisposition of a higher percentage of this type of fibers can increase their muscle size easily while those with a higher percentage of slow-twitch fibers have to train really hard to put on muscle. It’s the main reason why an elite sprinter genetically has a greater percentage of fast-twitch fibers than an elite marathoner, it all comes down to what we are born with.