The magnifying effect stops working when you put larger image and put it back to small.
 
You can see it in the demo


demo.flexiblewebdesign.com/lightmart/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=62&virtuemart_category_id=54

Hi,

It is not a bug or error. We did it that way to increase the performance and smoothness of enlarging. The zoom even must be re-run when the picture resolution is changed, re-running the zoom event affects the smoothness of enlarge function and causes performance drop.

So; when you "Enlarge" the product picture, the Zooming stops working (until you click an additional picture to re-run the Zoom event).

Regards,
support