they refused to exchange a tensioner

On the 21st of Aug I purchased a tensioner for a polo 1.9tdi. And upon fitting the tensioner? It broke, when I took it back for exchange as it wasn't the first time I bought parts from them. And i've never experienced any problems with the store or the branch itself. So it comes with huge disappointment that a store like Goldwagen would fail to exchange a part which could have been an acception , since they all not 100%. Very disappointed