VMs not in Write-Back mode because VirtuCache driver versions are not the same across hosts
VirtuCache driver versions across all hosts in the ESXi cluster have to be the same for VirtuCache to make peer connections between hosts. VirtuCache requires peer connections across hosts for replicating writes and for read redirects (after vmotion).
Typing this command (esxcli software vib list | grep vnx2) will tell you the VirtuCache driver version. The last four numbers preceding the .vib file name is the driver version.