Product type is not optimized
Ideally, you will want to import your variable products with its type set to
variable_optimized (an optimized product), but this may be restricted depending on the importing tools used.
For example, Product CSV Import Suite locks the product types to only allow those that come by default with WooCommerce; when you import a product using a different type, this importer will set its type to “Simple Product“.
To solve this issue, you will have to manually open the “Edit Product” page of every imported product, change its type on the product type dropdown and click the “Update” button. This will change the product type and make visible all the variations imported.