We have had this problem before, you will often get stains that only really stand out and show up when they are under artificial light. It's unlikely that you would see them when you left as the carpet would still have been damp and hidden shadow stains.
If it's a regular customer then you'll just have to deal with it, go back and treat it. Even though Chemdry have cleaned it in the past you say you have cleaned it for the last few times.
Unless you quantified with the client what the expectations of the clean were, ie: this mark is such and such and may not come out, then she will be expecting the carpets to be "cleaned" to her satifaction.
Neil,
Most customers are genuine and will only complain if they feel it is justified, treat them like that and you'll have no repeat business in a couple of years.
John, my thought exactly.