I recently painted my dining room using Dulux Roasted Red Matt Emulsion. An odd white bloom has appeared roughly a meter square halfway down one wall. It doesn't wash or wipe off and hasn't spread. I painted another couple of coats, no change. Tried Zinsser bin primer-sealer &stain killer, the spray version, gave it two coats and then two coats of paint, still there. Just tried Polycell one coat Damp Seal (as I'd run out of ideas!) and another coat of paint and it's still there. It's an internal wall, previously magnolia from when we moved in. The patch is in the middle so nothing rising or dripping down. Any ideas what it might be and how I can remedy it please. I tried taking a photo but it doesn't show.