  The accretion/ejection coupling in accreting black hole binaries has been
described by empirical relations between the X-ray/radio and
X-ray/optical-infrared luminosities. These correlations were initially thought
to be universal. However, recently many sources have been found to produce jets
that, given certain accretion-powered luminosities, are fainter than expected
from the earlier correlations. This shows that black holes with similar
accretion flows can produce a broad range of outflows in power, suggesting that
some other parameters might be tuning the accretion/ejection coupling. Recent
work has already shown that this jet power does not correlate with the reported
black hole spin measurements. Here we discuss whether fixed parameters of the
binary system, as well as the properties of the outburst, produce any effect on
the energy output in the jet. No obvious dependence is found. We also show that
there is no systematic variation of the slope of the radio:X-ray correlation
with normalization. We define a jet-toy model in which the bulk Lorentz factor
becomes larger than ~1 above ~0.1% of the Eddington luminosity. With this
model, if we assume random inclination angles which result in highly variable
boosting at large Eddington ratios, we are able to reproduce qualitatively the
scatter of the X-ray/radio correlation and the radio quiet population. However
the model seems to be at odds with some other observed properties of the
systems. We also compare the radio quiet black holes with the neutron stars. We
show that if a mass correction from the fundamental plane is applied, the
possibility that they are statistically indistinguishable in the X-ray:radio
plane can not be completely ruled out. This result suggests that some of the
outliers could actually be neutron stars, or that the disc-jet coupling in the
radio quiet black holes is more similar to the one in neutron stars.
