Emulsion has been applied to plasterboard and peeled/blistered.?

I have been told that this is because the top layer of primer has come off and then emulsion been applied to it. What should I do? Apply primer over the top of the emulsion section? the emulsion seems pretty solid apart from one section of the wall. Or should I dampen and peel the entire emulsion off and start again. Primer and then paint. HELP!