The app will only return rates for products belonging to the General Shipping Profile in Shopify AND when the product's location is part of the first location group within the General Profile.
If you only have one location/fulfillment service in your shop then you don't have to worry about this troubleshooting step.
Go to Shopify Admin > Settings > Locations
If only one location is listed, then there is no problem here.
If you do have more than one location, continue reading.
Possible Issue #1: If you have Locations in 'Not Shipping From'
Go to Shopify Admin > Settings > Shipping > General Profile > Manage Rates, and scroll down.
If you see a section 'Not Shipping From' with a location listed underneath, then when a product belonging to that location is in an order, no rates will show at checkout period.
So, you should click 'Add rates'
Choose 'Use Same Rates As' the existing location that is set-up, and click Done
Then you will see both locations grouped together into one 'Location group'.
If you have the app connected to this location group, then then app will be able to return rates for both locations.
Possible Issue #2: If you have more than one Location set-up with rates.
Go to Shopify Admin > Settings > Shipping > General Profile > Manage Rates, and scroll down.
If you see more than one section labeled 'Shipping to', then you may have an issue.
The app will only return rates for the first 'Shipping from' section.
The app will not return rates for any products belonging to a location within an additional 'Shipping from' section.
So, you can collapse all your locations into one group to fix this.
Next to the the location click 'Manage'
Then select Use same rates as, and then click Done.
Then you will see both locations grouped together into one 'Location group'.
If you have the app connected to this location group, then then app will be able to return rates for both locations.