I have a situation in my flat with leaking plumbing in the unit above. So a pretty serious mold problem has developed high up on my wall. (We're getting it fixed on Monday *fingers crossed* ) So this is another possibility for you to check into.

I'm no expert on mould, but I do know that bleach does kill mould. My guess is, the mould is growing within the plaster or cement, so a quick wipe with bleach kills it only on the external surface but it quickly grows back from within the substrate of the wall as soon as the chlorine has evaporated. Maybe you need to find a way to let the bleach soak into the wall more thoroughly? This is not coming from experience though - only an idea.

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