If you are having trouble finding custom attributes for one of your shipments within Shiptheory that should have been pulled from your Magento 2 store, a good thing to try is flushing the Magento Cache and Cache storage within your Magento2 dashboard settings.

You can access these settings by traversing the menu's as shown in the image below:

From here you should see an option that says Flush Cache Storage, click this option and that should be it, any new shipments from this point onwards should have their custom attributes allocated correctly.

If after this you're still having trouble please get in touch with support and we'll be happy to help!