Poor service from volkswagen

When I purchased my vehicle I bought a full maintainance plan from volkswagen. Towards the end of last year when my vehicle was serviced I told them to replace my wiper blades because it was smudging the windscreen when it rains and it's really bad at night. They told me that they cannot change the wipers. I really don't understand why because I purchased the maintenance plan and if something is wrong or not working properly it should be replaced. Last week I was almost in a serious accident purely because they didn't change my wipers. It was raining really heavy and the visibility was very poor.