I get the impression that in the Marvelverse, the Super Soldier Serum that created Captain America is like a Holy Grail that researchers keep looking for and failing to find. Hydra with the Red Skull and their Winter Soldier, Bruce Banner with the Hulk, Killian with Extremis... on it goes.
I'm beginning to suspect that one of the reasons that nobody is finding the serum is because (a) they are far too impatient, and (b) they're behaving like alchemists, not scientists.
Impatient, because they keep on leaping to human trials without enough data, often end up experimenting on themselves, and it all ends in tears.
Alchemists, because, related to the impatience, they change too many variables at once to be able to get any decent data, and they're left floundering around without having learned much.
I mean, seriously. If one was going to do proper research into the Super Soldier Serum, you'd start off with mouse trials. A heck of a lot of mouse trials. Because lab mice are bred to be genetically similar (so as to reduce variables), and you get your results faster because there's lots more mice to test on.
And if for some reason mouse trials are out because mice are too dissimilar to humans in this specific case, use chimpanzees! (Or pigs or cows or some other suitable animals).
One would carefully change one variable at a time, form theories, make predictions, test the predictions, disprove the theories, get as much data as one can, on a statistically significant number of results. That's the Scientific Method, folks. Form a theory and try to disprove it. Not "make a recipe and try to tweak it".
(Tony Stark is allowed to tweak things because he's an Engineer. Yes, he does Science, but he's mainly an Engineer. The most brilliantist engineer that ever engined.)
(Yes, I have a lot of Tony Stark love.)