Isoline milling works on a surface by surface basis. This can result in numerous retracts.
The orientation of the surfaces matters. Toolpaths are generated for surfaces whose normals point up. Surfaces are “auto flipped” where possible, but for vertical and some other cases, the machining side must be specified. Select the surface in the list box and click Switch machining side. Isoline milling may mill on the wrong side of the surface or if certain flags are set, it may skip the surface.
See Troubleshooting isoline milling