I had a similar issue, also left bag outside zipper. Took me 7 visits to different shops and nobody know how to fix it. Ultimately an old luggage repairman in the city center of Amsterdam knew just how to put in a fresh zipper. Only cost me 25,- if I am not mistaken.
Key was to cut a small opening at the start of the zipper to install a new zip-cart (no clue how to call these). The cart he used came from a travelers sleeping-bag which has 2-sided zippers and het cut one side off. After installing the new zip-cart we used superglue to fix the tiny opening again.