Uranium-235
Uranium-235 has different rules because it often is not in a pure form. Plutonium-239 is made in nuclear reactors by irradiating uranium-238 with neutrons, and uranium-233 is made the same way using thorium-232. Since they are different elements than the source material, they can be separated relatively easily through chemical differences. However, uranium-235 is produced from uranium ore, which contains 0.7% uranium-235 with most of the rest consisting of uranium-238. Since they are the same element, they behave in similar ways and must be separated by their slightly different masses. This is far more difficult than chemical separation, so varying levels of uranium-238 may remain after the first enrichment. If uranium is highly enriched, it can be used to make a nuclear weapon.
Read more about this topic: Special Nuclear Material