It's over training. Plain and simple. With the amount of work that the anterior and posterior delts get via compound movements on your other days, 24 working sets is just way too much. A dedicated "shoulder day" is retarded. Let alone one that is taking you 1.5 hours to complete. I never spend more than 50 minutes in the gym personally. If I can't get it done inside 50 minutes, it doesn't need doing. Work smarter, not harder. Particularly if you are natty.
I don't know if this is a troll thread or not, so I'm not going to spend a bunch of time. Try doing less for a while and see what you get. Focus on really clean reps paying extra attention to the eccentric portion of the movement. Concentrate on the mind/muscle connection. Try to "feel" the movement working the muscle. Short rest periods between sets (45 seconds to a minute). Use a weight that is challenging, but does not sacrifice form or your ability to concentrate on the eccentric portion of the movement. With the time you save, do some calf, ab or forearm work. Everyone I know neglects at least one of those areas.
Try lowering to 12-16 working sets and keep it between 6 and 10 reps. A press, a lateral movement, and a raise. Do it on a day that isn't near back/chest. Or try a push/pull/legs routine. If you are over training everything (and you probably are if that is really your delt routine), it could be a great routine for you.