Problem with handle bar

[UK]

Hey Group,

 

Like I had mentioned picked up a Hercules Impluso bike for my wife and off late I have been using it more than her. But couple of week back suddenly a problem came up. The handle bar became too loose and kept rocking here and there. Tried getting the handle bar tighten in couple of cycle store and looks like they don't have the right tool to fix it and it would require Allen key set. Just wanted to know is it a serous issue because I got it fixed from a 2 wheeler repair shop but came loose in just 10 min. The bike is still under warranty. What should I do. Should I get a Allen key set myself and keep fixing it?

 

Regards

 

UK