  Half of the radio afterglows for which there is a good temporal coverage
exhibit after 10 days from the burst a decay which is shallower than at optical
frequencies, contrary to what is expected within the simplest form of the
standard model of relativistic fireballs or jets. We investigate possible ways
to decouple the radio and optical decays. First, the radio and optical
emissions are assumed to arise from the same electron population and we allow
for either a time-varying slope of the power-law distribution of electron
energy or for time-varying microphysical parameters. Then we consider two
scenarios where the radio and optical emissions arise in distinct parts of the
  GRB outflow, either because the outflow has an angular structure or because
there is a long-lived reverse shock. We find that the only the last scenario is
compatible with the observations.
