No matter how strong you are, every single diplomatic treaty i've had does not work. In the end you have to pay to make them accept treaties. Even if they are very friendly to you or allied to you. It is ridiculous how bad diplomacy is on Rome II.
On my 3 play through so far i have suffered rejected proposal time and time again. My campaign as Rome, despite having powerful armies, conquering 3 provinces and having Syracuse as an ally i decided to start trading with them. But no, in the end i have to pay 5000 for them to accept the deal. In the end i didn't pay and decided to conquer them outright.
On my athens, Economic campaign. I was really annoyed by diplomacy. In the end i decided to halt the play through until CA fixes this mess.
And then on my other Egypt let's play i had the same issue. Diplomacy sucks...even Shogun 2 had better diplomacy than Rome II