Hi,
Yes, jQuery conflict issue is the biggest enemy of "not working" add to cart button.
If it has started recently, then it is 100% jQuery issue. Most likely the extension/module/plugin/component that you have installed to your website recently is tend to load an another jQuery library to your website which causes the Conflict.
While installing an another extension to your website, always check the source codes in your website. There should be only 1 jQuery loaded, and since the most important component in your website is Virtuemart and since Virtuemart officially requires jQuery1.6.4 (so do the tools [quicklook, zoom effect, and other things] that come with our VM Template), then your website MUST load jQuery1.6 only!. Eliminate the others.
Eliminate jQuery loading either by coding modification (in the extension that loads extra jQuery) OR look at this topic:
www.flexiblewebdesign.com/forum-virtuemart-2-template-puremart-support/523-the-easiest-way-to-solve-jquery-conflict-issues
Basically the plugin that explained in that topic above does the same thing what you are supposed to do. Leave 1 jquery in the website and blocks the others.
Regards.
Forum Support