[Mark Lane-Matthews]

Rust remover is very good on browning  stains if normal acid rinse does not work,but try a test area first.

[expro]

Is it a STAIN or is it Soil ?

If I have a "mark" appearing during or just after cleaning

I use a simple test to answer the above question.

Put down a little pre-spray on part of the"mark",

agitate in and then take some white toilet tissue
[several sheets] and place over the mark.

Press down hard on the tissue for about 20 seconds
 
then remove and inspect the tissue.

IF the tissue is soiled THEN the "mark" can be removed.

IF the tissue is not soiled and there is no transfer then

the "mark" is a permanent stain.

[Mark Lane-Matthews]

You would not always get a transfer as some stains would disappear through chemical reaction

[peter maybury]

What will work on one stain will not work on another. There are not many stains that cannot be removed it is knowing what to use on what. You do need to do an investigation, you will get no end of answers here. You need to look at what has happened in your situation.
Peter
www.carpetcleanercardiff.com