Sometimes products may not sync through REX into your eCommerce site. One of these reasons can be because the product is not yet enabled for sale.
To fix this issue, log into your RetailExpress portal and search for the product and open its "Edit Product" screen.
Scroll down to the bottom of the screen and check that the "Export to Web Service" box is checked, and that "ONLINE" and "WEB" are selected, as seen below.
Once these options have been selected, click on "Save Changes" and the product should synchronize within 1-2 hours.