I too have suffered from “flat chest syndrome”. It’s always been a primary focus of mine and I’ve found certain things that you have to do, and certain don’ts. Although my chest isn’t huge (I haven’t taken a recent and decent chest pic either), it definitely responded to what I’m about to say:
The problem with Bench Press with a barbell is that many people are more concerned about how much they can lift as apposed to perfect form.
If most BBs actually did the barbell bench press with proper BB form, they’d have to reduce the weight by 45lbs+. Not good for egos…
When I say perfect form, there is a difference between BODYBUILDING form, and powerlifting form. When you are aiming for size with the barbell bench press you need to make sure that your elbows aren’t tucked in, but rather that they are pointed out (thus taking your triceps out of the movement more). Also, you have to make sure that your back always stays flat on the bench. If you are lifting your back off it and flaring up your lats etc, you are taking the load off the chest. Someone seeking strength gains in the barbell bench press can easily make leaps in strength by practically putting their whole upper body into the movement…but this is not good for making the chest grow. Just because you lifted more weight doesn’t mean that your chest did much more work.
Another problem with benching is the rep range. Many people stick to just one range (usually the same range that they use for other muscle groups)…but you need to vary it from pretty high reps, to lower reps. The lowest rep range I’d ever go with on the barbell bench press is 6. Highest is 15. The chest is one of those muscle groups that you really need to feel the burn/pump in to see decent results (which means either high rep sets, or high set exercises).
The reason why I say vary the range is because some assistant muscle groups fail before the target muscle fails on some rep ranges, but not others. Take deadlift for example, some people’s grip fails before they can reach 8 reps (the grip doesn’t have as much endurance as the back). Therefore, some people stick to really low reps (e.g. 3-5) which means that the back fails at roughly the same time as the grip. The same sometimes applies to the chest, some assistant muscle groups fail before the chest does. Here you can also utilize pre fatigue exercises. You could tire out the chest muscle before you do the bench press so that the chest muscle is the one that definitely fails first. Or you could do post fatigue exercises.
One other neglected chest exercise is the decline bench press. This is a great exercise for targeting the lower chest (provided you focus on it). If I were to pick the best two exercises for overall chest development it would be Decline and Incline pressing.
When a muscle seems to be lagging behind, give it more volume by training it more frequently.
Obviously bear in mind too, that you really need to be bulking (taking in a very decent amount of calories) to begin to see a difference in size in any muscle - especially the chest. If your bodyweight isn’t going up, then it’s no use crying that your chest size isn’t either.