This is due to the surface being quite complex. Two approaches might resolve this, First, you can segment the curves and make multiple surfaces taht composes the surface area you want. Do not worry about the additional curves and surfaces created, these can be meshed across by removing the connecting curves (done automatically via the build topology functions). The second way is to create a larger surface over the top of the bounding curves. If you use build toplogy on this, it will cut the necessary surface out from the larger surface leaving the surface in two sections, the required surface inside and the redundant trimmed part of the surface outside. Simply delete the part of the surface not required.