Flip them over so theykick their legs. Try feeding off freshly molted white ones. Mine never refuse white dubia (and that's how I get them to start eating them too.)
I hold the cup that my cham eats out of so whenever the roaches arent moving, i shake the cup a bit. It scares my cham sometimes, but then she sees whats in the cup and goes right back to eating
They shouldn't be white after dusting. That being said, I've tried offering freshly molted dubia and ghost dubia. The Molted white ones were always chosen over the other kind. They know.