Hi there @Panos,
Welcome to the Community! Happy to have you here!
You did the right thing by first ensuring that your version of software and plugins are up to date and checking that all products are in stock! Do you mean to say that your products are published on your website, but not coming through your Klaviyo website? Have you run a historical sync, at the bottom of your integrations page? If so, and the products are still not synced to your Klaviyo account, I would reach out to our awesome support team as they will be able to identify more fully what the error might be and run a manual sync for your account, that will pull the correct products over!
However, if not all your products are published in your site, I would recommend publishing them on the backend of your website first, and then seeing if the products appear in your catalog. This could be one of the reason for products not showing up, as outlined in a similar post below!
Thanks for being a part of the Community!
-Taylor