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

You are on a beach, throughout the entire beach is one tablespoon’s worth of salt mixed in with the sand.

Your job is to collect all that salt from the sand, so you and some friends go picking through the sand in the beach with tweezers, individually inspecting every grain of sand to see if it’s a grain of salt. Such a process could take you and your friends their entire lifetimes.

Instead, you build a fantastically expensive machine that can separate the salt out of one shovelful of sand every 24 hours. You build a dozen of these machines, and with these machines, it will take you a year instead of your entire lifetimes to get that salt.

Replace the salt with enriched uranium, and the salt separator with the Uranium enrichment factories and that is an example of effort required.

The Uranium Enrichment factories cost billions of dollars and had to have a city built around them to run them, it was massively faster at making enriched uranuim than older methods.