This happend to me when I created a new md device and checked for the status in /proc/mdstat.
The problem is obviously just that the resync isn´t set in motion for some reason,
I played around with it, and maybe it even starts by itself, but if you are in a hurry,
you can tell mdadm to resync it right now by:

mdadm --readwrite /dev/mdN

 
Replace N with the md number and check /proc/mdstat, should do the trick.