Is your roofing metal? Does the "splotchy" occur during the colder months? Condensation can occur on the underside of metal roofing and then drip off, wetting the ceiling panels. And there is no easy cure, if that is the case.

I have the same ceilings in my bedrooms and could not paint them. such a strange material you paint it white and it looks wet, kind of grayish, I just left it since they were not in bad shape.

Most likely the gyp has dried out in spots. There is a permanent way to fix this.
First paint the ceiling with Kilz White Water-Based Interior/Exterior Primer, Sealer and Stain-Blocker. It will permanently block any stains and prevent any finish coat from soaking in. The finish is white so you may just choose to leave it without any further finish. About $21/gallon at Home Depot. Believe it's also available at Wal Mart.
A less expensive version is their oil-based version. I've used this on several projects and have never been disappointed. $16/gallon.

Here is a link that might be useful: Home Depot - Kilz