The success probability is generated by your actions that turn it seems. I don't mean that you can do certain things to increase it, what I mean is that if your assassin fails and you have keep reloading and retrying, he will keep failing. You'll have to reload and move some things around. Move an imam over there, use this army to siege that city, build this building, move that fleet over there, and sometimes you'll be able to reverse things and assassinate your target successfully.