The majority of People Don't Have Enough INTENSITY
Intensity is a factor in muscle growth. If you're not explosive, angry, or intense with your workouts, you won't gain a lot of muscle. Muscle comes from muscle damage, the micro-tears you create when working out. However, if you're doing the same repetitive routine, or not going harder than last time, you simply won't grow.
You will maintain muscle, but if you're wanting to grow, you need more stimulation.
Muscle growth will continue, as long if you're constantly stimulating and creating damage. Over time, your body will adapt to the workout intensity and conditions, so this is when people PLATEAU, or halt, on their bodybuilding/muscle-building progress.
In order to break this haul/plateau, you need more stimulation or some different kind of stimulation.
Our muscles contain different FIBERS which react to different stresses. Fast twitch and slow twitch fibers. In simple terms, you may need to target one of the either in order to achieve different types of stimulation.
To create more stimulation, your body needs to go through unfamiliar situations/events. You can lift more, heavier, longer, shorter, or use light weights to recruit different muscle fibers, along with focusing on mind-to-muscle connection and doing different styles of workouts. Instead of doing the same chest day on Monday, switch it up; mix up your routines, and shuffle them around. Add in different methods of training that particular muscle [group]. For example, instead of a biceps curl, try drag curling, or focusing on uni-lateral workouts. Meaning, instead if using Barbells all the time, which requires and involves both arms and biceps to perform the action, you're going to train each bicep individually. This will help cause more stimulation and growth because you're constantly focusing on that muscle rather than having both muscle groups perform that exercise. This in return also helps bring up lagging muscle groups because you can focus all of your intensity and effort on that particular muscle.
People stop growing because they're doing the same routines, styles, and movements, and won't go out of their way to grow. They're comfortable of what they're doing. SURE! it may be taxing on your body, leaving you tired and drained, but are you really seeing massive growth?
You need to go over the top. And I don't mean to work out for 5 hours and eat constantly. I mean, you need to confuse the body, and workout harder than last time, continue to eat healthily, and more when you can, but don't become obsessive/excessive with it.
Anyhow, Intensity is what grows muscle. People are too comfortable with their intensity output.
You're going to need to tax the muscle, go to failure, and go beyond of what you think of what is capable. You need to utilize that muscle that you're training to its fullest. You need to stimulate the muscle to a degree where you know, it has no choice but to repair and build new tissue.
Drop sets, super sets, pyramid sets, and partials, focusing on eccentric or concentric movements of the exercise. Either or, will stimulate the muscle to a degree..
Some people just lift to maintain health and power. Some people don't want to go to that extent to build muscle or to continuously build muscle. Some people do not want to endure the pain and agony that comes along with it. And that's okay. But if you're telling yourself, why am I not growing - well, you now know why.