Mercedes Benz Century City poor service

I am so disappointed at Mercedes Benz Century city. took my car in for service on 08 February and collected on the Friday the 9th. I just noticed now that my windshield washer is pumping slow. I went to the garage because i thought it ran out of water but to my surprise the water is still fine. I didn't have this problem prior taking my car to Mercedes for services. so obviously they did something during service. now I'm questioning if the service was even done properly. I paid a lot of money for this poor service and I want them to be accountable for this. i need someone to contact me and fix this urgently