Why uranium enrichment was slow/near impossible in the beginning of Manhattan project but few years later it became trivial?

Question

So I’ve watched Oppenheimer and a recurring theme in the movie was how tedious task it was to get fission material (demonstrated as filling a large fish bowl by marbles). All they manage to collect in two year period was just enough for two bombs. but fast forward few years US have a complete arsenal of bombs to flat the earth. I understand they must’ve innovated a method and the exact method maybe classified, what i’m interested in is knowing what was the obstacle(s) for this and rough idea of how they might’ve overcome it.

In: 450

Answer 1

There were some improvements on the processes but it’s mostly that they’d finished building a significant quantity of infrastructure to produce the fissile materials.

Also I think you’re assuming the US had a much larger nuclear arsenal sooner than it did, post Hiroshima/Nagasaki the US had the capability of producing 3 Fatman equivalents a month but had 300 by 1950, ~2k in 1955 and then it starts to get silly in the ’60’s.