Copy and paste entire song in GarageBand for iPad
Maybe your project is too big? I have heard that GarageBand, even on a Mac, has limitations when the project gets too big. It would refuse an operation or just hang or crash without much feedback. Try with a much smaller/simpler test song and see if you can accomplish what you want. Or try posting exactly the steps you do to copy and paste, and exactly what happens (what GarageBand does) after.