Linden Court Damp issues

I manage a block of 14 flats in a converted and extended (1990) Edwardian house in central Bournemouth. Despite having some major renovations earlier in the year, some of our flats in both the old and the newer part of the building are reporting damp issues. The building is brick and render, the newer part has cavity walls and the older part solid walls, to the best of my knowledge. I have had an inspection of the interior of the roof space, which I am advised is water tight. One of our leaseholders in a top (2nd floor) flat in the newer part has developed severe issues floor to ceiling in the bedroom over the last couple of weeks. I believe that this could be a consequence of a repair which was made to the gulley and gable immediately above the worst of the damp earlier this year. We also have an issue with Flat 5, which is situated on the ground floor in the old part of the building.