Everything has to be very precisely shaped. You need extremely sophisticated fuses that can go off within miliseconds of each other to ensure the plutonium comes together to reach super-criticality or whatever it's called when the nuclear chain reaction becomes self sustaining. If the time is off even by a tiny amount you get a normal explosion and a liquid jet of super-hot plutonium instead of a nuclear explosion.

That said, the actual nuclear detonation isn't that complicated, it's like high school or early college level physics. Once people figured it all out, once we knew it was possible, the core concepts aren't that hard. These days building a nuke is an engineering challenge, metallurgy and machining. The physics is well understood.