According to my second-year textbook, "when prolonged CS treatment is stopped, the adrenal is atrophic and unresponsive and the pt is at risk for acute adrenal insufficiency. The pituitary may not be able to secrete normal amounts of ACTH for as long as a month..." So it sounds like it is a problem with both ACTH production and the adrenal's ability to respond to it.
Hope this helps