I am trying to get a custom destop icon to be displayed for my app but for some reason no matter what I do the same default java icon shows up. I have tried everything I can
There's an example of how to do this in the JavaFX in Action book, if you have access to that. You need to make sure the res/icon64x64.png file is actually downloadable from whatever site the app is hosted on, relative to the JNLP's location. Try loading it directly in a browser to ensure its available/valid.