If any of products have any "custom field", then the "add to cart" button is disappeared in GRID VIEW (in LIST VIEW add to cart button is exist, except "generic child cart variant" custom field type though). We did this on purpose due to some complains, it also made sense to us. If you were aware, original Virtuemart doesn't have "add to cart" button in category browse page, and this is the purpose why they didn't add it by default (to avoid adding product to cart in case there is some custom fields (variations).
So consider, if any product has any custom field(s), people would add the product without being aware of the variations. (size, color etc..) So it is better to make them click the product, see the product details page in order to see the possible variations.
But if any product has no custom field at all, then the "add to cart" button will appear in category browse page.
This is not a bug or failure. Everything in our VM Template has been developed on purpose to avoid any possible confusions and give better shopping experience to your buyers.
Regards.
Forum Support.