With reference to Why aren't there more supersoldiers?
After the formula to the serum used on Steve was lost, many tried to replicate it over the decades without success. In the accepted answer to that question, it was mentioned that the serum used on Bucky was the closest attempt made: it granted the same qualities Steve has to Bucky but without transforming his physical appearance.
Strictly speaking, nobody is looking for Erskine's exact formula. They are looking to recreate its effect of transforming ordinary people into the perfect human and supersoldier. It stands to reason that all attempts have failed because the test subjects after their corresponding experiments did not achieve the desired perfection observed in Steve.
The serum used on Bucky, once again, is considered the closest attempt to success but is ultimately still a failure. That probably means, in Hydra's eyes, that Bucky is not as perfect a soldier as Steve. It can't be because of his mental brainwashing—Hydra did that, wanted that, and it works as intended - nor his lost arm - humans never possessed the ability to regenerate limbs in the first place—which aren't caused by the serum used on him.
If Bucky is considered to have the same qualities as Steve, then exactly how is the effects of the serum used on Bucky different such that Hydra rejected it?
The only answer I've right now is that Bucky's physical appearance wasn't transformed like Steve, although their physical abilities are supposedly the same. Is that what Hydra wanted? I would imagine not standing out with an extraordinarily perfect body build is preferred for a soldier-assassin.