Cant Integrate AI in a PU

[Valtegro]

Can somebody tell me under what circumstances a lesser union partner (and AI) will say no to integration.

In my case its me Ayutthaya that cant integrate Sukothai.
Is it because I switched to Sunni and Sukothai is still Buddhist?

 

[AndrewT]

Please post this in the appropriate game's main forum, thanks.