Without going too detailed, a 100% D-efficient design is typically only possible with an orthogonal, balanced, and unconstrained design space. When restrictions are introduced, a 100% efficient design is generally not possible because the constraints reduce the volume of the information matrix relative to the ideal (unconstrained) case, resulting in lower efficiency.
Your situation calls for a split-plot design.
"All models are wrong, some are useful" G.E.P. Box
