Some years ago a friend pointed out that it is easier to boot Raspberry Pi images from an NFS instead heaving the entire Linux sit on the SD. The benefit is that on the server machine everything can be stored on a SSD or hard drive, whatever you prefer.
My problem was that when I had a power outage the Raspberry Pis won't boot anymore because the couldn't access the NFS machine. I was looking for long for a solution and didn't find anything. The service was up and running and the /etc/exportfs showed the right directories.
Then I switched from XBMC to OSMC and then I came across a posting at their forum. The solution is actually simple: Adding the nfs-kernel-server service to restart. Once I added service nfs-kernel-server restart to /etc/rc.local everything was fine.