In general, the reusable parts of fuel are long-lived and the parts that complicate reuse of spent fuel are short-lived.  In general, fuel becomes easier to reprocess over time, largely because less shielding is needed to protect against radiation from the decay of short-lived fission products.  One exception is the decay of Pu-241 (with a half-life of 14 years) to Am-241 (half-life 432 years).  Though the decay rate is much lower, the radiation hazard is higher because the decay includes a low-energy gamma ray.  Recycled fuel with higher Am-241 is therefore somewhat harder to handle safely.  Also, the Am-241 is not fissile and is therefore prone to absorb neutrons (in a standard light-water reactor) and produce higher transuranics rather than fission and produce energy and fission products, so the spent fuel is also harder to handle.

Fast reactors in principle can help reduce these problems, since most transuranics undergo fission in a fast neutron spectrum.  But fast reactors do not have a strong track record of reliability and carry a significant cost premium.  So in considering the economics of recycling, it's important to take into account not only the cost premium of the fuel but also the cost premium of the reactor needed to use the recycled fuel most effectively.