Why Is My Erlang Test Suite No Longer Able to Call External Functions

I'm having a bit of an issue now with my test suite. I've not changed any of the code that it's now complaining about.

It's no longer able to see a function that's being called from within the test suite module that exist outside of it in a .hrl/header file.

example_module:example_function failed Reason: undef

Can anyone this of as to why this has happened? I'm aware of meck and the possibility to mock the function call but since it was working before and has suddenly "stopped working"...

Regards, Snelly.