Weiss VW ****ming me with the 15k km service

I am very disappointed with the service that I received from Weiss VW in Durban CBD. I sent my car in March for a 15k km service and was told that the service had already been done and they only had to remove the service notification. Today I sent my car for the 30k km service and afterwards I was told that I missed my 15k km service. This means that VW is no longer liable under the warranty because I apparently missed the service. They also now do not know who removed the service notification on my car therefore leading me to believe that the service was indeed done. This is absolutely ridiculous. I need them to rectify this as this was not my doing but their employees who failed to do their job.