If the product is in the "Under Review" section of jet for a long time then it may be because all the details of the product might not have been entered.
Although it takes approximately 1 week for the products to go from "Under Review" section to Live Mode to availbale for purchase.
As per jet.com guidelines for product listing each product must have atleast one jet attribute value( it is not mandatory to enter all jet attribute value for each product).
This way jet will receive all the values for all the attributes.
Jet need some product properties by which product can be configure with other products by using jet attribute values.
Sometimes the products get stored in their backend but never get listed on frontend as this is Jet.com's internal issue which when gets resolved, the products start showing up at jet.com live panel.