There are many potential reasons, but I am going to keep it simple and give you just two. The first and most common reason people suffer from underactive glutes or “glute amnesia” is due to lifestyle. Even when people train hard every day, if they spend the majority of the remainder of the day sitting down, then they are simply not using their glutes. And remember the old saying - if you don’t use it, you lose it. Unfortunately, this is just what happens with your glutes.
In the modern bodybuilding industry, the term "professional" generally means a bodybuilder who has won qualifying competitions as an amateur and has earned a "pro card" from their respective organization. Professionals earn the right to compete in competitions that include monetary prizes. A pro card also prohibits the athlete from competing in federations other than the one from which they have received the pro card. Depending on the level of success, these bodybuilders may receive monetary compensation from sponsors, much like athletes in other sports.
Another common reason I see glutes that aren’t working properly is due to injury. Often an injury happens that changes the mechanics and motor programming of a person’s body. This can lead to some muscle groups becoming overactive, while others become underactive (think: compensation). This can alter things for a long time without the person even knowing it.
There are many camps within the weight training fitness community. We have bodybuilders, Crossfit athletes, powerlifters, Olympic lifters, and strongman athletes just to name the most popular ones off the top of my head. One thing they all have in common is that they all use resistance to achieve a particular goal. They all also “share” particular exercises. Most resistance-training athletes do barbell squats, overhead presses and deadlifts. I can write pages of differences between each of the disciplines I listed above and I can also write quite a bit about their similarities but one form of resistance training is MORE different than the others. Bodybuilding is the only sport that judges the appearance of the athlete rather than their performance. This may be why bodybuilders tend to get poked at the most.
Most typical bodybuilding programs have way too many sets and reps and use the wrong exercises. However, if you lower the total volume, go heavier, and use compound movements as I’ve outlined above, there is nothing wrong with a body-part split for advanced lifters. In fact, it’s often less stressful to the joints than your average upper/lower split.
^ Burd, Nicholas A.; Yang, Yifan; Moore, Daniel R.; Tang, Jason E.; Tarnopolsky, Mark A.; Phillips, Stuart M. (2012). "Greater stimulation of myofibrillar protein synthesis with ingestion of whey protein isolate v. Micellar casein at rest and after resistance exercise in elderly men". British Journal of Nutrition. 108 (6): 958–62. doi:10.1017/S0007114511006271. PMID 22289570.
How to do it: Use just your bodyweight, dumbbells, or a resistance band around the top of your knee to activate your glute medius, Reames says. Assume the same setup of a basic squat, feet shoulder-width apart and knees neutral. Squat down, knees bent at 90° angles, and step to the side. Continue repeating this side-step motion down and return to your starting position.
Note that this recommendation is for total weekly volume, which means it would need to be divided up based on how many times you’re training each muscle group per week. So, for example, someone training everything twice per week would do 30-70 reps for each bigger muscle group in each of those workouts, and 15-35 reps for each smaller muscle group in each of those workouts.