How to repair a loose coving/cornice?

Here's my random question. In my mother's room, there is a coving, the bit of decoration made from plaster, that you attach to the top of the wall, where the wall meets the ceiling. One of the pieces is in danger of falling off. What exactly do I need to do? Would polyfilla lack the appropriate adhesive qualities? I would imagine so... so what exactly do I need to get in order to fix this for my mother? Thanks in advance.