oh, I'm sorry you are going through this. You probably got no responses previously because most Givenchy gals and boys have little to no problems with their pieces.
If you still have the bag, could you take it to the store you purchased from for an exchange? Or at least advice on where to fix it? Maybe a message to Givenchy corporate (who owns Givenchy anyway? think there was a thread about it somewhere - a tPFer contacted the company that owns/sells in the Middle East last year).
Your other option is always a trusted cobbler who may be able to tighten the stitching.