Our 50s brick house with fibrous plaster ceilings has small cracks in either the top edge or bottom edge of the cornices in most rooms.

I have attempted on a few occasions to patch them up using good old no more gaps, which works for a while, but the cracks eventually open up again.

I am wondering is this due to a bad bond between the cornice and the brick/render/plaster walls ?
Was the wrong adhesive possibly used during construction ? The original owner was pretty hands on during construction, and did most things well, but might have messed this part up.
My other theory is that the ceilings are flexing up and down with pressure caused by wind action ( tiled roof is not sarked ) since the plaster sheets are held up by fibrous plaster loops (don't know the correct term).

I have attached some photos that show the problem, the colour are a bit strange due to some image adjustments to make the cracks more visible.

What is the best way to solve this once and for all ?
If it involves removing and re-installing the cornices, would a special adhesive be required, or would standard modern cornice adhesive bond well to plaster ?

It would be nice to stop the shower of roof dust around the edges or the room, particularly the ones that have been done up.

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