If you have cleaned mould from your walls and watched it return within weeks, you are not doing anything wrong. The mould is returning because its cause has not been addressed — only its appearance.
Condensation mould grows on cold surfaces. When warm indoor air meets a wall surface that is below the dew point temperature, moisture from the air condenses on that surface. Mould spores feed on that moisture and grow. The surface is cold because the wall has poor thermal resistance — heat escapes through it freely, leaving the interior surface cold.
Cleaning kills the spores on the surface. The wall surface temperature does not change. Condensation forms again on the next cold night. Mould returns. This cycle continues indefinitely unless the thermal performance of the wall is improved.
Anti-mould paint contains biocides that suppress mould growth at the surface. In mild cases this provides temporary relief. The biocide degrades over time — typically within one to three years — and the mould returns. The wall is still cold.
ProofTherm insulating plaster and ThermoSilicone insulating render both achieve this by creating a thermal barrier that reduces heat loss through the wall and raises the interior surface temperature. The elimination of mould is a direct and permanent consequence of improved thermal performance.