My mother describes a three step approach to any dispute.
You ask nicely. You try to make them like you and understand why you need what you're asking for. You appeal to the basic human need to think of themselves as a good person.
Don't escalate yet. You're still being charming. You're just not taking the first no as an end to the matter. If you have to, hint at stage 3.
Your life until now has been about accumulating resources to destroy this person. You will sacrifice your first born just to lower the property values on their parent's home. When Alzheimer's has stripped all memories of your beautiful wife, you will still remember this goddamned dispute. This is your purpose.
Most issues are solved at 1. Stages 1-2 should be exhausted. Also, you need to be able to hear their position. It's about being willing to go stage 3, even if you never do in your entire life. People need to sense that you're willing to go there if they're unreasonable