Form Function works from Homepage, but not Forms

I have a "SSWA plsql function" form function which is intended to open an external link. The form function works as expected when I click on it in the Oracle homepage however an error occurs when trying to open the function from within forms.

The error is "Function not available to this responsibility. Please contact ..."

Do you know why this issue occurs, and what changes may be required to ensure the link opens via forms as well as the homepage.