Have you make some BIOS upgrade?
In this case be sure that all hosts have the same BIOS version and same configuration for VT and NX.
Another solution could be create a new cluster with EVC enabled.
iirc, there's a note in one of the release notes about an upgrade putting cpu masking bits into the .vmx of some guests. You will need to shutdown the VM to clear them out, though, so shutdown a test vm that won't migrate and reset to default the mask of the vm, then power it back on on the same host and try to migrate again. If that's the problem, then you know what you are facing. You are stuck powering off the VM's to get past this problem.