Most drywall damage is cosmetic and easy: a dent, a doorknob hole, a crack at a corner. Fill it, sand it, prime it, paint it. But some damage is a symptom of a problem behind the wall, and painting over it just hides the evidence until it gets expensive.
The first sign is softness. If a section of wall or ceiling gives when you press it, or feels damp, there's water getting in from somewhere: a roof leak, a window that isn't flashed right, or a pipe in the wall. The drywall has to come out so the source can be found and fixed, and often the framing behind it needs attention too.
The second is staining that comes back after you paint it. Brown or yellow rings on a ceiling that reappear mean the leak is active. The third is cracks that follow a straight line along a seam and keep reopening, which usually points to framing that's moving or drywall that was never fastened properly.
The fourth is nail pops across a whole wall rather than one or two, often from lumber that dried and shrank after the house was built. The fifth is bulging or sagging, especially on ceilings, which can mean the board is too thin for the joist spacing or has absorbed moisture over years.
Any of these means the right fix is to open the wall, correct what's behind it, and hang and finish new drywall. That's a bigger job than a patch, but it's a job you do once. We're happy to come look and tell you honestly which kind of repair you're dealing with.