I have had a "subject" for a while, with the integration option showing a date to mark the 10 year period. Once the period was elapsed, the option is still unavailable saying simply that I can't integrate it.
Does it have to do with ethos? I have collectivist, militarist, and xenophile.
Best Answer
As you said where you originally post your question, your subject was not a vassal but a protectorate. It has nothing to do with ethos.
Protectorate can not be integrated into your empire. However, when they reach 40% of your technology, they are converted into vassal and can now be integrated.
Be careful, if you get a new protectorate in mid-game, they will probably never be able to reach the 40% as they start with basic tech, even if you give them free research boost.