Why Is My Drywall On Ceiling Sagging After I Hung It?

Hello, Week and a half ago, we hung 3/8" drywall over the popcorn ceiling in our porch room. The porch room, which wasn't completely level to begin with was added as an extension to the house long before we moved in. We found joists, they are roughly 24" apart, and I threw a lot of screws up there... show more Hello,

Week and a half ago, we hung 3/8" drywall over the popcorn ceiling in our porch room. The porch room, which wasn't completely level to begin with was added as an extension to the house long before we moved in. We found joists, they are roughly 24" apart, and I threw a lot of screws up there but in between the sheets of drywall that we hung vertically, the drywall is dipping, sagging, waving, etc. in a few spots. Is this because we used too light of drywall for a ceiling that has 24" joists apart from each other or is it because I didn't properly secure enough screws? What do you think would cause this and what do you recommend?

Thank you so much
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