generative modeling for Rhino
I think it would be helpful if you post definition and 3dm file so the question can be better understood.
Normally this is due to the anchor points not being coincident with the mesh vertices. Anchors need to be not only intersecting the mesh but also exactly on the loaction of mesh vertices.
There is a way to make sure that anchor points are valid. If is doesn't matter that ancors are exactly where you place them, use the ClosestPoint with all the mesh vertices and the original anchors. This selects the nearest mesh vertices to your original anchor points.
You can also move the nearest mesh vertex towards the the orignal anchor location, reinsert the moved point and rebuild the mesh.
Just use kangaroo settings -> tolerance :) (not sure if anchor points will move)