I had this same issue, the back mic was working fine but the front was not working for the Fractal R3.
Whenever I plugged in the mic, I would get a Realtek HD audio manager popup. I noticed that the playback volume was at 0% when I plugged it in the front vs 100% when I would plug it in the back. Looks like the settings are specific to each port. Adjusting it fixed the issue for me.
Hope this helps.