Hi,
Keep this in mind.
FashionMart is a template for Virtuemart, which usually all about CSS and styling the DIV container and elements to make things look good. In very rare situations template overriding effects negatively while Virtuemart functions. (that usually happens when the new Virtuemart is released and our template is not compatible with that new version, but this is not the case now) Also; this kind of issues won't happen "sometimes", it will happen "always" until our Template gets patched and become compatible with that new VM version.
Adding product to cart successfully or removing it from there, have absolutely nothing to do with our template. Those events are taken care by the Virtuemart Core files (vmprices.js)
You are also saying, "sometimes" the Fashionmart CSS doesn't get loaded. This simply doesn't make any sense. Because in your screenshots, I am still seeing the "Switch View" icon and in the shopping cart, i am seeing the brown background. Those come with the FashionMart CSS file, if the CSS wasn't loaded, you wouldn't see those elements as well.
Get a "firebug" plugin and investigate the source and CSS files when you see that green add to cart button to find out why those green add to cart button still gets shown off.
Because i just opened your website and i am seeing everything looks just fine.
Regards.
Support.