Application starts on OSX but not Windows?

I’m experimenting with Elixir on Windows – and I have an app that I can run using iex -S mix on OSX.

When I open it on my Windows 10 box, if I use the same command, I get the iex repl but the application doesn’t start. From the repl, I can then start the application manually and it’s fine.

Any idea what I am missing in my files or installation?