True dynamic portal sorting isn't really possible in FileMaker. Instead one has to simulate the effect with multiple tabs/layouts or use a calculation field that, with limitations, can serve as the sort field by referencing different portal fields depending on what the user has selected.
We'd need to know what method you are using and exactly how you've implemented it to be able to help you.
Thanks that's what I needed to know. I'll just make several identical layouts that have the various portal sortings, that'll be easy enough.