one way to bring up a 'weak' body part is to add some very light frequency to that muscle group with either 'active recovery' or what some call 'cell swelling' work.
example :
-- if you destroyed your chest on Thursday, then when you come in for leg day on monody, before you begin that leg day , you start with 8 sets of something like cable flies for chest. this is only moderate intensity , its just to bring blood flow and nutrients to the chest, you don't even go close to failure. this helps your chest 'actively recover' and grow (it adds more volume without adding a bunch of fatigue) then go hit you leg workout.
example : cell swell work . before you hit your chest on thursday , on weds after your back workout or tues after shoulders, go to something like the incline hammer strength bench press . load it up with very very light weight. Your goal is to get 100 reps. if you get 100 reps non stop, then thats way too light.. you'll prob want to hit about 35 or so reps and reach a point where you need to take 15 seconds rest. then keep going, then rest, then keep going . until you hit the 100 reps. this is light enough weight that your not going to tax your chest muscles before your upcoming chest day, what it will do is drive a ton of blood to the muscle (metabolite training essentialy) then glycogen restore with carbs afterwards, and drive the chest full of glycogen stores (hince 'cell swelling'), and your chest is going to be full and ready for that big chest day. you might have just a slight amount of musce soreness in your chest when you do hit that chest day from this cell swelling work , BUT, thats a good thing , it will actually help with the mind muscle connection on your compound chest movements like bench press, you'll better be able to feel your chest working and be able to contract the muscle better , and thus more growth
just my 2 cents worth of a tip