1. You're a "routine" person.
Once our body adapts to a workout routine, it will naturally stop growing or become stronger. Your mass and strength will then be stagnant. Try changing your sets of workout and add in more variety!
2. You're not sleeping enough.
Our muscles repair and recover while we are sleeping! Actually it's not just our muscles, our organs and our entire body repairs and recover while we are sleeping. Try to have a minimum of 6.5 hours of sleep daily. 8 hours will be perfect but some of us might not have that luxury unfortunately. Also, the "golden period" where our body does the repair work will be from 12 am - 5 am. Note that sleeping too late is a No Go as well.
3. Diet. (Food and Drinks) We all know protein is a must to buff up. Right. But are you taking enough carbohydrates? Carbohydrates are just as important as protein! We will need them Before and After our workouts for Gains!! Also, do not neglect the greens! Make sure you're having enough water daily. On average, an adult will need a minimum of 2 litres of water daily. 60% of our body is water. And yes we need protein to build muscles. But they need even more water!