  A cosmological model that aims at solving the coincidence problem should show
that dark energy and dark matter follow the same scaling solution from some
time onward. At the same time, the model should contain a sufficiently long
matter-dominated epoch that takes place before acceleration in order to
guarantee a decelerated epoch and structure formation. So a successful
cosmological model requires the occurrence of a sequence of epochs, namely a
radiation era, a matter-dominated era and a final accelerated scaling attractor
with $\Omega_{\phi} \simeq 0.7$. In this paper we derive the generic form of a
scalar-field Lagrangian that possesses scaling solutions in the case where the
coupling $Q$ between dark energy and dark matter is a free function of the
field $\phi$. We then show, rather surprisingly, that the aforementioned
sequence of epochs cannot occur for a vast class of generalized coupled scalar
field Lagrangians that includes, to our knowledge, all scaling models in the
current literature.
