They are falling because of water damage and they need to be tossed, Have you checked the insulation above for moisture?
Far better to go with drywall when filled and painted you would have a better air seal which really is a must.Insulation is all brand new from roof repair several years ago. Doesn't seem to be any moisture. just trying to repair the tiles. would it be possible to throw plywood instead of tiles? trying to fix for as cheap as possible...