When I had a bedroom extension built the ceiling height was increased by moving it further into the roof space. Now the plaster has fallen off wherever there is a nail on the sloping section between the wall and the ceiling and when I try to fill it the filler falls out. How can I get this to stick? Any advice is greatly appreciated

try pressing plasterboard up if there is movement you will need to stick in another nail or screw to hold it tightly to ceiling. that may solve your problem of filler falling out.