We find sufficient conditions for the construction of vertex algebraic intertwining operators, among generalized Verma modules for an affine Lie algebra gˆ, from g-module homomorphisms. When g=sl2, these results extend previous joint work with J. Yang, but the method used here is different. Here, we construct intertwining operators by solving Knizhnik-Zamolodchikov equations for three-point correlation functions associated to gˆ, and we identify obstructions to the construction arising from the possible non-existence of series solutions having a prescribed form.