Not putting the muscles under enough time under tension.
-Time under tension is whats gonna make the muscle grow, you really have to connect with your mind and makin sure you're contracting the muscles through the whole mouvement, making sure you're using the right muscles.

Not going throught the full range of motion a muscles allow you to do into an exercise.
- You will need the help of google or youtube on this one, every muscle group has it's own way of contracting and stretching. If you follow the instructions and go through the full range of motion during an exercise, you will use more muscle fibers and you will get a better pump.Which will lead to bigger , better , stronger muscles.
 

Not doing a mind to muscles connections when contracting the muscles.

-If you don't constantly make a mind to muscle connection, you're wasting you time in the gym. You don't even know what muscles or what part of this muscles you're training, and you might even be using another muscle than the muscle you're suppose to be using in that exercise.

 

Not increasing weights, or reps often enough.
- To make your muscles grow, you need to tell him that you've got something heavier coming in next week, you need to make your muscles adapt to the circumstences, in order to make it grow bigger. Think about it, if you did 100 pounds last week, and this week you decide to try 105 pounds, you're muscles witll react to it, realizing that you've decided to go heavier, they will have to grow in size or strenght to compensate.

 

Using too much weights for your abilities.

- This is a comon mistakes amongs the beginners, you will see guys putting way too much weights on the bars trying to swing some reps so they can tell everyone they did X amount of weight for X number of reps.
Sacrificing form for heavy weights is the worst mistakes out there. You're using other muscles and lots of momentum to move a weight you couldn't normally do. Remember guys, we're bodybuilders not weight lifters, you wanna be able to make the muscles grow, who cares if you can rep 2 plates by bouncing it off your chest and only going half way up? The muscles won't grow and you won't get any stronger so what's the point of that? think about it.