Dog urine

Sometimes you just can't get the yellow out 100%.

Sodium Perborate - this is a mild oxidizing bleach.
Hydrogen Peroxide - this is also an oxidizing bleach.

You can purchase products with these ingredients blended into them or purchase them in their own form.

What type of cleaning agent are you using?

What type of carpet are you cleaning?

Can't be of much help if I don't have any info to start with

It is possible that you will not be able to completely remove the discoloring.  Especially when the urine spot has been there for a long period of time and it is on a nylon carpet(sometimes customers just let things go too long).  If urine is left for to long some of the color will eventually get set into the fibers.  When this happens it will be near impossible to extract all of the coloring from the spot because it has essentially become a STAIN.  You would have to use a more "Harsh" and "Risky" method of removing it which would mean some kind of spotter that reallies on a chemical reaction to change,remove, or destroy the color.  Of course this also means that you run the risk of taking the original dye out of the carpet also leaving you worse off than you were to begin with(and remember most carpet fiber doesn't start out white before manufacturing).  So if it were MY customer and all but a slight discoloring still remained I would explain the risks of trying a different method of removal and let them decide whether to leave it be or not.   

Make sure your spotting agent is leaning towards the acid side of the Ph scale or you will have a heck of a time trying to get it out.

If it's on a polypropylene carpet than you're probably just using the wrong cleaner(normally it will come out with just ordinary carpet detergent and some elbow grease).  This particular carpet won't hold a stain because of the way the dye is introduced into the fiber during the manufacturing process.