By default, Yusp imports items and refreshes product information in its database when a product is clicked on on your website. During this step, Yusp imports the first couple of products.
If you have traffic on your site, you will see the progress bar moving. However, if you’re installing in a test or development environment with no traffic, you will have to go to your site and click on some products to facilitate the import (you’ll need to click on 5 at least, in order to proceed to the next step).
If imports get stuck at 1 or 2 products, it is likely, that it is a product ID issue. Revisit the "Define a product ID" step in the installation guide and also revise the appropriate section of this documentation to check if you’ve done everything right.