Could be one of many reasons for this,
The protection treatment was not applied correctly.
The treatment has warn off, how old is the suite ?
Cotton / viscose fabrics appear to suffer quite badly from dye of off certain clothing, causing discolouring which is mostly impossible to clean up. Try Kens idea, this will lighten up the fabric.
The suite has not been maintained regularly enough for the amount of soiling it receives, sometimes people get the wrong type of fabric for there needs. This is not necessarily their fault as they are not informed enough to make that decision.
You may not have cleaned it properly
. Discount the last one, I was only joking with ya.