If its normal then they shouldnt sell this stuff, cardboard would have lasted longer.
Yes new underlay. 9mm. As it happens the worn/dodgy bit has occured on the only part of the house where the fitter also layed the underlay. Its only a small area so I didnt mention it in the origional post. The rest went down a few weeks before the fitter came as I had estate agents in to give a value on the property.
I can almost guarantee that the carpet is not wearing out, what you are describing is almost certainly a pressure mark, NOT wear. The appearance has changed due to the way light is reflecting off the pile, it has nothing to do with wearing out.
If the carpet was wearing out, it would appear threadbare and tufts etc would have come out of the carpet and worn away, even the cheapest of the cheap carpet won't wear out in 10 days.
Assuming its pressure, this will have no effect on how hard wearing the carpet is, it's purely a visual phenomena, the carpet will wear just fine. All carpets will suffer from pressure, it just so happens that polypropylene is particular susceptible to showing pile pressure for a number of reasons however, it's highly unlikely that the carpet is faulty or wearing out.
It's well worth having a constructive chat with the retailer and discussing the issue, but be prepared for a similar answer to what I have put above.
Retailers will be well versed in inspecting pressure shading in carpet and will normally agree to inspect and if necessary ask a manufacturer to cast their eye over it too. Reputable retailers will happily discuss the issue further, for instance, under the above circumstances, if my customer felt they had made a mistake in choice of carpet, I'd offer an allowance towards re-selecting another type of carpet, but I'd be looking to cover the costs of the original carpet, I.e. I wouldn't replace it out of the goodness of my heart for free, but I would offer the customer the opportunity to limit their losses, your likely to need to cover the costs of fitting again though in full.