Niantic has updated an official known issues support page for Pokémon GO. Read on below to learn more:
Gift Exchange Known Issues
Mateo does not always appear at Gyms.
Issue description: When approaching a Gym at the end of a Route, Mateo does not always appear. Tapping on the Gym will prompt Mateo to appear for a Gift Exchange.
Issue status: Investigating
Having the maximum amount of Pokémon in storage blocks Trainers from completing a Gift Exchange.
Issue description: Trainers who have reached their maximum number of Pokémon are blocked from completing a Gift Exchange. In the meantime increasing the number of Pokémon you can hold or sending one to Professor Willow will allow you to proceed.
Issue status: Investigating
Some Gifts do not appear when tapping on Mateo a second time.
Issue description: After entering, then exiting, and then re-entering the Gift Exchange dialogue with Mateo nearby Gifts no longer appear on the Gift menu.
Issue status: Investigating
Quitting a Route may prevent Mateo from appearing
Issue description: Quitting a Route without finishing it may prevent Mateo from appearing in subsequent Routes, even if you didn’t encounter him before quitting.
Issue status: Investigating
Source: Official Pokémon GO support page
Source: Pokemon