Alternatively....if it is a fairly old carpet, maybe draught marks appearing from floorboards that have built up over the years.
These are not always apparent before cleaning, in particular if the carpet was badly soiled thus hiding the lines initially.
If they are extensive draught lines, then you may have a problem removing and in fact convincing customer that this is what they are.
Possibly lift carpet to see whether there are signs of draught marks showing on carpet back or on underlay, this would go part way to convincing the client.
If it is just blocked jets then just a thorough reclean should do it.
Best of luck.