Sometimes if they`ve only just or recently captured you, it needs you to log out and in again before the server recognises you as enslaved, even if you are already paying tribute.
what this means to you it that sometimes if you can`t break free and you are already 10 times his pr and doing all the right things, you will need to log out and in again before it will allow you to do so.
What i usually do in that situation is open a different tab or window from the one that i`m about to break free in and log out there, then all i have to do is click the link to declare independence once i log in again from that other window/tab, saves you a few seconds only, but that can be precious time if the guy hasn`t yet noticed you have climbed higher in pr.