  Abridged:
  The decay of half of the GRB radio afterglows with long temporal monitoring
is significantly slower than at optical frequencies, contrary to what is
expected in the simplest fireball model. We investigate four ways to decouple
the radio and optical decays: an evolving index of the power-law distribution
of the shock-accelerated electrons, the presence of a spectral break between
the two domains, a structured outflow, and a long-lived reverse shock
contribution to the afterglow emission. [...]
  In the fourth scenario, it is assumed that the radio afterglow emission
arises in the reverse shock propagating in a steady stream of ejecta, which
catch up with the decelerating GRB remnant. This scenario can accommodate the
properties of the afterglows with slow radio decays and requires that the
injected energy is less than or comparable to the initial fireball energy,
while other afterglow parameters have reasonable values.
  For a jet, the transition to a semi-relativistic motion mitigates the radio
decay, however this scenario would work only when the slower radio decay is
observed well after the steeper optical fall-off. A structured outflow with a
relativistic core, yielding a fast decaying optical emission, and a
non-relativistic envelope, producing a slowly falling-off radio afterglow, is
not a viable solution, as it fails to decouple the radio and optical decays.
