When people do weight training sometimes they work body parts in the wrong order. It is important to realize that working out bigger muscles and then move to smaller muscles will enhance your workouts.
For example, when I do an upper body workout I will start with chest or back because those muscles are the biggest and are needed in order to workout the smaller muscles. I will do pushups, benchpress or other chest exercises and then move on to biceps or triceps. That is because if you workout the biceps or triceps first you won't be able to use those muscles later on in your workout because they will be tired due to being smaller muscles.
Anyone who has worked out biceps or triceps first and then try to do a chest workout will know that you won't be able to lift as much or do as many repetitions as you normally would. This is because the muscles became tired from working them out first and did not have enough energy left in them to help fully for the chest workouts.
If you want to get a good workout start with the bigger muscles and then work out the smaller muscles. You will have a better workout overall and be able to see improvement over time.