Hi Phil,
A subfuncton cannot be an entry point for MATLAB Coder. You'd need to move that subfunction into a separate .m file, then the App will pick it up.
If this is truly a subfunction (not a nested function) then such a move should be painless, you literally cut-n-paste the function body into a .m file with the same name as that function name. The original code will continue to work (it'll find the function in the .m file in the same folder).