The problem is that some products do not show the product navigator, and other ones does.- :-\
(http://zurweb.net/ejemplo.jpg)
http://forum.virtuemart.net/index.php?topic=79799.0
VM 2.0.14 and Joomla! 2.5.8
1. Prev/next product is only shown if there is more than one product in a category.
2. Looks like you are using differenr template overrides for those two products.