Blistering Emulsion on Chimney Breast

Any ideas how I can stop emulsion from blistering? It's Valspar washable / scrubbable matt. The blistering has only appeared on the chimney breast wall. Picture attached.

please post a pic with a coin or hand to show scale.

How old is the house, and the plaster?

How was the wall previously decorated; and how did you prepare it for painting?

Were there any watermarks or brown stain?

Is the chimney ventilated top and bottom?

I've since rubbed down the blisters. The dark colour in the photo is the underlying paint. There are about 10 similar sized blister patches on the wall. The house is 1930s. I've been in it since 2007. Until about 2010 the wall had painted wallpaper. I suspect it was done not long before I moved in to hide problems. I recall having similar problems decorating back them and seeing damp patches. Some work was done on the chimney and that seemed to improve things. My son painted the wall a dark grey in about 2015 and again I recall some minor blistering. This time I rubbed down the entire wall and filled any imperfections with a fine surface filler. The blistering is more widespread and paint peeled off in a couple of places. There's a working gas fire in place so the chimney is fully ventilated. I think some repairs have been done to the chimney breast in the past as the sections immediately either side of the fire are plasterboard rather than brick.