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I don't think it is 'damp' damp.
When the room gets steamy, having a shower etc, this bit of the ceiling does seem to get more condensation on it, I wondered if there was a metal bracket embedded in the ceiling in that area or something.
We are top floor so fingers crossed it's not dampness coming through from outside...
Edit: Just been and checked and it feels bone dry right now. The ceiling does sound pretty hollow there though. The entire ceiling in the bathroom looks pretty terrible actually, looks like it's been patched and filled in numerous places.
Will get around to repainting at some point and see how it lasts.
M_V
The main causes are moisture or grease on the surface, you should check that the area is not damp, it looks from the photo as if it could be.
If it's not damp then you could prime it with an oil based primer like zinsser cover all, then paint emulsion on top.