We use AC-susceptibility measurements to study the dynamic response of vortices in a Pb thin film with a periodic array of antidots. In the linear regime, from the field dependence of the AC-penetration depth lambda we determine the curvature of the pinning potential well alpha for different matching fields. For applied fields H below the first matching field H-1, alpha reaches its highest value due to the strong vortex localization at the pinning sites. For H-1 < H < H-2, the metallic-like behavior of interstitial vortices gives rise to a reduction of alpha and a further decrease of the pinning strength is observed for H-2 < H < H-3. However, for H-3 < H < H-4, a reentrance in the pinning strength appears, due to a specific configuration of the flux line lattice which strongly restricts the mobility of vortices.