1. Not resting enough. 

It's important to not only rest your muscles groups around 24-48 hours between training sessions to see increases in muscle mass and strength, it's really important to get enough sleep as well. While you sleep your body is turning cells over and regenerating, if you don't have enough of that "power off" time, your body won't be regenerating as quickly and results will be slower.

2. Not eating enough/Eating the wrong things.

 I see this a lot where individuals won't eat enough to fuel themselves for their workouts and therefore are unable to push themselves hard enough. Another thing is not eating enough of the right things, if you're trying to gain muscle you need 1.2-1.8grams of protein for every kg you weigh. So for me I need 115-127 g of protein a day to gain muscle, which is actually a lot. 

3. Too much too soon.

 A lot of people want to start off working out by going hard, going from not working out at all to working out 5x a week. Chances are you're going to be so sore you won't be able to make those workouts and will feel discouraged. You need to slowly build your way into it, aim for 2-3x a week for a phase of 4 weeks, then if you're feeling good ramp it up! 

4. Exercise ADD 

Meaning, doing a little of this and a little of that, if you're not consistent your body will never experience change, you need to train your body in the same movements to see those get better/stronger. If you're too ADD in your workouts you'll never adapt and see results specific to that form of exercise. 

5. Comparing yourself to the person next to you.

 You have no idea where they're at in their journey, and even if you're at the same point, unfortunately our genetics are all very different. Some people gain muscle mass very easily, and for others it's much more difficult.