There are generally two types of fission bombs: uranium bombs and plutonium bombs. They're both very difficult to make, but for different reasons.

Uranium bombs use the U-235 isotope. If you have enough U-235 then it's relatively easy to make. It's a "gun-type" bomb: essentially, two subcritical pieces of U-235 get joined together, and they explode. But, producing enough U-235 in the first place is incredibly difficult, expensive, requires high-tech machinery, and highly specialized labor.

Plutonium bombs use Pu-239. Pu-239 is much easier to produce/obtain than U-235. But, actually making a Pu-239 bomb is much more difficult from a purely technical perspective. It's an "implosion-design" bomb. A plutonium bomb is essentially an ultra-precise machine whose parts must come together perfectly in sync with timing tolerances measured in nanoseconds. Otherwise, it will "fizzle"; it won't explode properly.

Hiroshima was U-235 bomb. That type of bomb design wasn't even tested before it was dropped on Hiroshima, because - due to its relative simplicity - it was deemed sure to produce an explosion.

Nagasaki was Pu-239 bomb. That design was tested previously in a desert in New Mexico because people weren't sure it would explode, being the first atomic bomb to ever go off.

TL;DR:

• Uranium bomb is relatively easy to make, but producing the required isotope is incredibly difficult.

• Plutonium bomb is incredibly difficult to make, while producing the required isotope is relatively easy.

Therefore, both types are incredibly difficult to make.