Not able to "actually" activate a font
Fonts automatically disappeared in my illustrator very often and I was not able to call them back.
(Very frustrating issues, especially in ongoing projects...)
I tried to activate a font through Adobe fonts website and Creative Cloud many times, but both of them failed.
(My computer, illustrator, and creative cloud apps are all new and new updated.)
What happened often was that I activated the whole family of fonts, but only some of them was activated.
For example, I tried to activate "Questa Sans" Family with 10 fonts included, but only 9 of them were actually activated and able to use in illustrator. "Questa Sans Regular" was not able to activate after plenty of tries.
(The weird thing was that it showed only 8 of them in creative cloud...)
It will be very helpful if you fix it.

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Chiu-Ying Wang commented
It's somehow fixed later on, but still a frustrating problem, very waste of time...
OS/device: Mac mini (Apple M4, 2024), MacOS Sequoia 15.5
I found it happened much more often on my new Mac mini, comparing to my old MacBook Pro, 2016 with older version OS.
At one time, after I restarted InDesign and opened the project that I just closed, the fonts disappeared. And I was not able to add them back in the moment.
It usually happened when the file was closed for some time, but not this time. -
Thank you for reporting this issue. Can you confirm which OS/device you were seeing this problem on?