Every atom has energy in it, regardless of whether it is radioactive or not. Radioactiveness just makes it relatively easy to extract that energy. But even then, it's not that simple, not every radioactive material is good for a nuclear reactor. If the fuel absorbs too many neutrons without fission, or produces elements that do, then it can become poison for the reactor. And if it, or the elements it produces, emit very few delayed neutrons and very quickly then it makes it harder to keep the reactor in a sub-critical state (i.e. it makes it harder to not make it explore). Often for these reasons you can't fully use reprocessed fuel, and instead you have to mix it in low percentages with normal fuel. Reprocessed fuel is also harder (thus cost more) to produce since you have to work with highly radioactive materials.