Why does AirDrop only sometimes require me to open the AirDrop window on my desktop?
According to your devices, it should work reliably. But Apple documentation makes two relevant observations:


Find out if the Mac has “Block all incoming connections” turned on in Security &amp; Privacy preferences. A Mac won't receive items using AirDrop if this setting is turned on. 


If AirDrop on the receiving device is set up to receive items from contacts only, make sure that both devices are signed in to iCloud. Also make sure that the email address or phone number associated with your Apple ID is in the Contacts app of the receiving device.



If your Mac Pro is located on a safe environment, I would go to Finder > AirDrop and set it to accept files from anyone. It made my devices find each other much faster in my experience. It might do the trick for you.