How do I stop my ceiling paint peeling when I paint it?

On emulsioning my ceiling, patches of the original paint have cracked and /or peeled away. I scraped back the local area to paint that I believed was still well bonded and sealed the edges and bare plaster area before repainting. Either the surrounding area then cracks and peels or other patches occur. It would appear from the chalky residue on the bare plaster that it hadn't been suitably sealed. I think this may be true of the whole ceiling but as I've said when I scrape back I come to paint that appears well bonded. Any help would be good thanks. If I have to take all the paint off and seal the entire ceiling does anyone know how to remove the "bonded" paint easily? Thanks for any help you can give.