Not all cases are (or should be) covered in the sample tests, that's why they're samples ;)
I would try using methods better suited for this job (like I did).

Definetly it's a problem with your code, try copying this test in the sample test tab:

dotest 3, ["oint", "wh", "ple"], ["you", "would", "a", "versioning;", "your", "me", "answer", "using", "have", "here", "for", "ruby-doc.", "In", "neither", "pointed", "I", "browse", "you", "1.9?", "I", "am", "updated", "most", "out", "glad", "what", "I", "input", "that", "your", "does"], ["oint", "wh"]
You can see the expected answer is ok, oint and wh are present in pointed and what.
That reduce + map doesn't look ok.
I'm not so well versed in Elixir to say you exactly where in your code is the problem, my code seems simpler to me and passed all the tests. Maybe someone else can give you better help than me.

I just solved it in Elixir with no problem, could you be more specific?