  We explore the dynamic response of vortex lines in a Pb thin film with a
periodic array of antidots by means of ac-susceptibility measurements. For low
drive field amplitudes, within the Campbell regime, vortex motion is of
intra-valley type and the penetration depth is related to the curvature of the
pinning potential well, $\alpha$. For dc-fields below the first matching field
$H_1$, $\alpha$ reaches its highest value associated with a Mott Insulator-like
phase where vortex lines are strongly localized at the pinning sites. For
$H_1<H_{dc}<H_2$, the response is mainly due to the interstitial vortices and
$\alpha$ drops to smaller values as expected for this metallic-like regime.
Strikingly, for $H_2<H_{dc}<H_3$, we observe that $\alpha$ reduces further
down. However, for $H_3<H_{dc}<H_4$, a reentrance in the pinning strength is
observed, due to a specific configuration of the flux line lattice which
strongly restricts the mobility of vortices. We present a possible explanation
for the measured $\alpha(H_{dc})$ dependence based on the different flux line
lattice configurations.
