There's lots that could be at play here. Is your function (now a method, if I understand correctly) in the same class? Are you using dynamic-function if it's a method now (and thus you should be using DYNAMIC-INVOKE)? Does the function return character? What's the error number you're getting?
I created an overly simplified class to show how to achieve this in a simple way:
class myclass:
method public void main():
MESSAGE dynamic-invoke(this-object, 'fnBeheersgroep',1,2)
VIEW-AS ALERT-BOX INFO BUTTONS OK.
end method.
method public integer fnBeheersgroep (input nr1 as integer, input nr2 as integer):
return nr1.
end method.
CONSTRUCTOR myclass ( ):
main().
END CONSTRUCTOR.
end class.
Using it as a function rather than a method gave me error 13664:
Cannot invoke method '' in object of type '' using DYNAMIC-FUNCTION(). (13664)
Hope it helps.