Are both clusters running hardware with the same CPU brand? Intel or AMD I mean by that. The migration of VM's between clusters is supported, as long as they share the same datacenter and (in example) CPU brand. EVC can't do magic and only works within a certain brand of CPU's

If you click migrate, and then choose the destination, it should tell you why it isn't able to vMotion (not live migration, but that's just to be punctual) it to the destination cluster.

Hope this helps,