Alternatively, Just try to "Disconnect" and "Re-Connect" the affected hosts first; before removing them from the cluster and even the vCenter all together and then adding them again, which is quite a tedious task if you have plenty of hosts that are affected.
I had this same problem today, AGAIN! I had it a couple of times in the past and I used to fix it by removing the host from vCenter, then adding it again. Lukily i found this post: DRS invocation not completed
All credit to "Rapidparts, Inc" who posted on that blog and mentioned to Disconnect and Re-Connect the affected hosts. I tried it and has worked for me on vCenter 5.1.
It seems like this is quite a common and recurring problem and seems to have started with vSphere 5.0... Does anyone know if VMWare is aware of this or may have addressed this problem already? There is not much on this problem to be found on the Internet and especially on VMWare's site.
P.S.
There is a similar issue in vCenter 5.x where DRS may still be working but the performance graphs show: "Performance data is currently not available for this entity" - This can also be temporarily fixed by Disconnect\Reconnect of the affected host. A permanent Fix for this issue would be nice 
(Removing hosts and adding them is not it - the issue keeps creeping in every so often).