Most probably for the second material the production order is not generated (yet).
At the moment I can think of only a few possible reasons in a standard system when the field is greyed out - subsequent documents, rejection reason, delivery block with confirmation reset, credit block (because it should also block quantity confirmation). Because most of these work at document save, for newly added items you still can change the requirement type before saving.
In case the orders in your PRD environment already have production orders generated, I am afraid this would be a challenging task.