Two things: (1) You need a special nuclear reactor in order to breed U-238 into Pu-239. Ordinary reactors will breed a spectrum of plutonium isotopes, but the Pu-239 needs to be isolated for nuclear weapon purposes, so that's another isotope separation problem. (2) You need to master the implosion bomb technology. With U-235, you can use a "gun assembly" approach, such as on the Hiroshima "Little Boy" device. But Pu-239 is far too sensitive to a fission chain reaction for that to work without the device going off prematurely at low yield.
Two things: (1) You need a special nuclear reactor in order to breed U-238 into Pu-239. Ordinary reactors will breed a spectrum of plutonium isotopes, but the Pu-239 needs to be isolated for nuclear weapon purposes, so that's another isotope separation problem. (2) You need to master the implosion bomb technology. With U-235, you can use a "gun assembly" approach, such as on the Hiroshima "Little Boy" device. But Pu-239 is far too sensitive to a fission chain reaction for that to work without the device going off prematurely at low yield.