Cracked veneers

I have had 8 lumineers fitted 2yrs ago, in the first year, 2 of these chipped, in the second year another one has chipped all when flossing I have flossed very carefully, but it was always difficult once the veneers were fitted, now I have a black stain appeared between the veneer and tooth on front tooth, I have to go back to the dentist to see what this is, the last chip is very noticable and the dentist says it will cost £480 to replace. Taking in to account the timescale that these problems have occured do you think it reasonable for the dentist to charge or do you think she should replace them free? Thank you I woul be interested in your opinion.