You don’t have to be an expert in exercise science or body mechanics to take an objective look at yourself every month and identify some of your weaknesses. We all have muscle weakness, whether we notice it while performing a lift, or looking at ourselves in the mirror. No matter your level, working on your weaknesses can make you more symmetrical, which can go a long way towards helping you achieve the look you want, getting stronger, and helping you stay injury-free. Here are some examples of some common weaknesses I have seen

Having issues staying upright during deadlifts, squats or lunges = weak core

Waist looking thick even though you are relatively lean = add some mass to shoulders and lats

Trouble with descent of bar, or stability on bench press = weak lats or biceps

Achy knees = weak hips, glutes, hamstrings

Small arms = maybe you are focusing too much on biceps, bring up your triceps; they are a larger muscle group

Besides an all-encompassing program that should have you improving on all of this, a good way I have found to bring up these weaknesses is through frequency training. So if you decide you want to bring up your triceps, pick a certain amount of pushups and perform them every day for a month. If you want to add some mass to your calves, do some kind of calf raises every day for a period of time. The frequency with help bring up those weaknesses, and can go a long towards your progression in the gym.