Give them their money back
as if,
Well thats what most seem to adveritise, " Money back if not completly happy "
" Cleanest you will ever see, or money back "
" We will make your carpets like new " yeh course you will
Explain to them the possible reasons for the custys dissapointments , ie traffic lane grey, light reflection, condition of carpet etc etc, but of course you should really be making all of these problems clear to your custy on surveying, explain the expected results , the chance of complete removal of stains and you wont get to many dissapointed custys.
i always try to give them an honest evaluation, then when it turns out better than their expectatons its a bonus.
Geoff