ROOF LINING Problem?!!

[Euphoria]

I recently took my 03' Boxster S to my local dealer for an oil seperator change. I went to recieve my car today and noticed the interior roof lining (Headliner) has gone loose on both sides in the areas above the driver and passnger's shoulder to the extent that i am able to see the convertible top's steel structure.

My dealer argued that:

1) I can't prove that this problem happened while the the car was in their possession.
2) Even if it happened while the car was with them, they wouldn't claim responsibilty because in his opinion this is due to normal wear & tear.

Ok, so this brings me to the following questions:

1) Is it common for the roof lining in boxsters to go loose?
2) Does regular use of the convertible top affect the roof lining?
3) Is it possible to damage the roof lining when accessing the engine?
4) What would cause the headliner to loose tension ???

Any info relating to the above will be highly appretiated, and please note that i couldn't spot any specific place where the lining has detached. It seems to have simply lost the tension which normaly keeps it in place.

Thanks in advance for any info you can provide.