Can you please include screenshots of your shipping configuration? Without seeing your setup, it's impossible to know why rates aren't returning.
From the limited context, it sounds like you haven't configured your shipping to include all cart combinations. For example, if you charge based on weight, you may only have weight-based pricing configured for up to 10lbs, but the order in the cart may be 10.1lbs or higher. If it isn't configured, it cannot calculate a rate since the system wouldn't know what to do.