The short answer
Carpet mold returns when the conditions that allowed it to grow are still present. Mold needs moisture. The EPA states plainly that if you clean up mold but do not fix the water problem, the mold will most likely come back.
Cleaning the visible surface of a carpet does nothing about a damp pad, a wet subfloor, or a slow leak behind a wall.
Common hidden moisture sources
Slow plumbing leaks under sinks, behind toilets, or inside walls can keep the edge of a carpet damp for months. Window and door leaks wet carpet along exterior walls. In basements, concrete slabs can wick moisture up into pad and backing.
Indoor humidity is another quiet cause. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. Rooms that stay above that range can support growth in carpet without any leak at all.
Cleaning reaches only part of the carpet
Carpet is layered: face fibers, primary backing, secondary backing, then pad. Most cleaning reaches the top layers. Mold often grows on the backing and pad, which is exactly where cleaning is weakest.
Adding water during cleaning can also make things worse if the pad and subfloor do not dry fully afterward.
What a lasting fix looks like
A lasting result starts with finding and correcting the moisture source. Then affected porous materials, which the EPA notes may need to be discarded, are removed. The structure is dried before anything new goes down.
If your carpet mold has returned once already, that is a strong sign that removal and moisture correction are needed instead of another round of cleaning.
When to call
If a musty smell has returned after cleaning, or stains keep reappearing, call (855) 661 4275 Monday to Saturday, 8am to 7pm, to talk through what you are seeing.
This guide is general information about carpet mold based on publicly available EPA guidance. It is not a substitute for an assessment of your property or medical advice.