This plugin returns shipping rates. When an item is added to the cart, the rate is returned. If the visitor goes back to the shop and adds another item to the cart, the plugin no longer returns the rate.
When the cache is cleared, the shipping rates are provided again. The plugin developer is saying that this is an issue with the theme.
Please make the full backup of your site as well as the Theme settings export and check the issue on the Storefront theme to detect if our theme causes the problem. Storefront is a free theme developed by WooСommerce.
I reproduced with the StoreFront theme twice this morning but cannot seem to do it again. I may have had an address entered incorrectly. Right now it does not look like the issue is happening with the StoreFront theme.