it's not plaster, but something like skim you see on walls that is added onto the original concrete ceiling. that piece an area is loose from the concrete, so when you knock it sounded hollow and if you use abit force you can peel off that layer. problem is when you peel off the loose area you drag the surrounding area too making the problem area bigger.

i don't think it's humid, maybe they did not apply properly or when they work on it they pushed a spot out or use too much water till it doesn't stick.

not sure if temperature causing expand & shrink will cause this also but when i try to remove a small spot the surrounding gets affected and comes loose too just like orange skin. anyway to prevent surrounding being affected?

if i leave it like that i figure gravity will continue to pull it away from the ceiling. if remove is alot of area. that area will be plastered with ceiling but i don't want them to start flaking off inside the plaster ceiling.

so how did you fix yours?

mine is 1 spot the rest is stick on ceiling, but if i try to peel off the loose one i drag the surrounding with it too.

when you peel off the area won't you spread the area to surrounding?