There are several reasons why muscle growth can slow to a crawl, but here's one most people never think about: you've stopped getting stronger.
When you first start training, watching the weight on the bar go up each week is a great feeling. The problem is, once these "newbie gains" wane, most people stop putting an emphasis on strength and start wasting time with so-called advanced protocols that typically leave them spinning their wheels.
Until you reach an advanced level of physique development, your sole focus should be on developing strength in exercises that suit your mechanics, over a wide variety of rep ranges.
Remember, if you're lifting the same weights you were three years ago, chances are you probably look exactly the same. If you've lost your focus on this single most important factor in building muscle, use this method of goal setting to get you back on track.
Pick 2-3 exercises that suit your mechanics in each of the following movement patterns:
Once you've picked your exercises, test your 6-8 and 8-12 rep maxes in each of them. Then train hard for 12 weeks and focus on progressing the lifts as much as possible while maintaining perfect form.
In a world full of fancy training techniques and "cutting edge tricks" to build muscle, it's easy to forget that progression is the foundation of the hypertrophy pyramid. Once you keep this at the forefront, you'll start growing again.