hi,
It is not a problem. It is a feature.
If you have a define a "custom field" for the product, the "add to cart" will become inactive, the text of "Add to Cart" will be replaced with "Choose Variant", and the button becomes a link that goes to product details, where visitor select variant and add the product to cart. It is for perfect UX, nobody will purchase any wrong product from your website.
If the product doesn't have any custom field, then everything will work just fine and products will be added from category by clicking the "Add to Cart" button
We developed it that way based on the feedbacks, because otherwise people were adding the products to cart without being aware there are product attribute (size, color etc..) and because Category page has a very limited space, there is no way to put those attribute select boxes to category grid box.
If you check the category page on our CleanMart Live Demo:
demo.flexiblewebdesign.com/cleanmart/
it works the same.
Regards