I live on the 2nd floor of a 3 story condo building, built in 2000-2001. Over the last few months, I have noticed that some of the drywall is sagging. It isn't sagging in the middle of the sheets, but at the edges. In the right light and looking at the right angle, I can see the lines all 4 sides of some of the sheets. It's difficult to tell what is causing this, as the ceilings are popcorn finish. I just bought the unit one year ago and this was certainly not noticeable at that time. It started with being able to see the line edge one 4' end of a sheet, but now I'm seeing it at different places in the unit. All of this has happened in 6 months time and there are now at least 6-8 places where I can see at least one line edge. Any thoughts on the cause? Could it be that when the ceiling was hung, the screws broke the paper and now the sheets are slipping down off the screws? Could this be structural? Thanks for your input.