If you are the project manager and those resources report to you, then delegate the task and outline your expectations of what you need, how you want it, and when you want it, and if you're ignored, take action.

Do developers really think they are so special they cannot be assigned an ad hoc task...by their boss?

If you are the project manager and those resources report to you, then delegate the task and outline your expectations of what you need, how you want it, and when you want it, and if you're ignored, take action.