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

What would happen if you used non-enriched uranium or plutonium? Would the bomb be far less effective, or not react at all?