  We have studied the pinning force density Fp of YNi_2B_2C superconductors for
various field orientations. We observe anisotropies both between the c-axis and
the basal plane and within the plane, that cannot be explained by usual mass
anisotropy. For magnetic field $H \parallel c$, the reorientation structural
transition in the vortex lattice due to nonlocality, which occurs at a field
$H_1 \sim 1kOe$, manifests itself as a kink in Fp(H). When $H \bot c$, Fp is
much larger and has a quite different H dependence, indicating that other
pinning mechanisms are present. In this case the signature of nonlocal effects
is the presence of a fourfold periodicity of Fp within the basal plane.
