i think your completely missing the point. Pairing different muscle groups if you are doing the same split same sets/reps isnt going to do anything significant. Why not try a routine where you are hitting bodyparts more than once a week instead of beating the shit out of them once a week!?!? The answer has been obvious for a while, you are either
A. overtraining (cardio everyday, too many sets)
B. You change exercises way too often
C. You think about everything way too much
we all know you know about diet, so its not that.
send a message to the muscle, it will grow, let it rest, then send it again. Theres no magic here. People say "well arnold did high volume blah blah" but he ALSO worked each bodypart more than once a week, point being, if doing that kind of split isnt working for you anymore, CHANGE it!! THE WHOLE ROUTINE, NOT which bodyparts are paired together while still hitting the muscle once a week. Seriously try it, you might surprise yourself.