  Hindered magnetic dipole transitions $\Upsilon(nS)\to \gamma \eta_b(n'S)$ are
studied in the covariant light-front approach. Compared with the allowed
magnetic dipole transitions, we find that results for hindered magnetic dipole
transitions are sensitive to heavy quark mass and shape parameters of the
light-front wave functions. It is possible to tune the parameters so that the
predictions of branching fractions of $\Upsilon(2S,3S)\to\gamma\eta_b$ are
consistent with the recent experimental data, but the relevant decay constant
of $\eta_b$ is much smaller than that of $\Upsilon(1S)$. We also generalize the
investigation to the charmonium sector and find the the same conclusion.
