It has happened to me before too! I really want to know how to fix it as well, I'm on a UK game and at some point in the game, I just couldn't raise or lower my relationship with countries either, it has become very annoying, I did try 1 thing though.
I would just declare war on the country and make peace right after that, being as big as I am now in the game, countries accept right away, and then I'm able to raise and lower my relation with that nation, but it seems that after a while that also stops working, so its a temporary solution that cant really be used for much.