Vacuum daily in high traffic areas.
Weekly in other areas.
Try a dry chemical clean.
Wet cleaning is especially difficult with berber because it really hangs onto the soap residue.
Soap and Water - Detergent Residue
Wet carpet cleaning methods, including some that contain the word "dry", can leave detergent to dry in carpet. After carpet is cleaned with a wet carpet cleaning method the water evaporates from the cleaning solution and the detergent dries and re-crystallizes within and throughout carpet fibers. During the drying time, (2 hours to 6 days), the water (H20) evaporates from the "cleaning solution" and the detergent remains. Detergent residue can attract new soil, trigger rapid re-soiling and cause carpets to "ugly-out" and become matted down. Simply put, detergent residue (soap) attracts new soil.
If it is not a high quality type to start with, new carpet may be the only solution (wool is better than nylon).
Berber is the hardest carpet to keep clean and the hardest to get clean. It draws dirt to the bottom of the fibers and traps it there and because of the loop weave and the twist of the loop being larger at the top than the bottom, it's very difficult to extract with a vacuum cleaner or a even a professional wet extractor including truck mounted equipment.
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