I am also experiencing an issue with the app unable to retrieve any tracking data for a valid Waybill number for the courier DHL Express International.
I can clearly view tracking data using the dhl.com (track: express) website.
I am on Deliveries version: 5.7.14
UPDATE [25/Oct/2021] 22:53 - After a few hours from creation of new delivery item in app, and leaving this item saved in a pending state, and with no tracking details, and still set to the courier type DHL Express International, the app finally returned the tracking details.
Not sure why obtaining the tracking info was delayed for a few hours in appearing in the app automatically in the first place. Strange. Patience does it in this instance it seems.