Pavel has asked me to assist you with this issue.
To try to fix the problem, I recommend you to use the Update ATUM's stock control tool to set all your products as Controlled by ATUM. Here you can find two articles that could help you:
Update ATUM's stock control tool
How to display your products in Stock Central
If nothing of that worked, you will need to check if all the product's data is saved properly in the database after importing. Most of the cases the product type field is missed or not supported by ATUM.
ATUM only supports the following product types: default WooCommerce product types, WooCommerce Product Bundles, WooCommerce Subscriptions, and WooCoomerce Bookings.
I hope this helps,