Hi mnixon,
If the person is having trouble initiating a glut max contraction, I would have a look at what other inhibitors are present.
If someone has a tendency to drop into an anteriorly tilted pelvic position, they can sometimes find it difficult to isolate glute contraction. It might be worth looking at their Rec Ab strength + Lumbar extensor / Hip flexor mobility?
You mention that hamstrings are dominant and strong, what is their hamstring ROM like?