How to refix loose plaster coving to the wall/ceiling?

I have recently had coving added to my 1970's semi-detatched house. All the newly applied plaster coving was fine for the first 4weeks or so but now an area of around 1meter long (not near a corner) has come loose from the ceiling and the wall. The affected area is at a joint between a 2.4 meter length and a cut length. The coving is still fixed to the wall and ceiling but is slowly flaking off the paint and has a degree of play when gently pushing. Whats the best way to refix this without having to take down the entire length?

Thanks