[plt-scheme] Why would the order of functions matter?

Dear All,

I have a student getting the following error:

Welcome to DrScheme, version 4.1.1 [3m].
Language: Beginning Student with List Abbreviations; memory limit: 128
megabytes.
reference to an identifier before its definition: insert-everywhere-in-a-word
>

The questioned identifier is defined as a function after its first
reference. When I re-arrange the function definitions DrScheme no
longer complains. What can be causing this?

-- 

Cheers,

Marco