For vMotion to work the vm's have to be within the same datacenter. If they are not, you have to do a cold migration. Some requirements to check:
- Ensure the VM is not connected to a CD-Rom or Floppy Drive
- Cannot be clustered with another vm
- Both ESXi servers must use same share storage
- Both ESXi must be connected via gig or better
- The two servers must have compatible cpus
- access to same physical network
These are some of the requirements. To understand them all include vmotion storage, check out the 5.x Admin guide. Take a look at virtual machine management/migration and migration with vmotion. By reviewing these, you will be able to see what the issue is. Here is the URL to the guide, VMware vSphere 5.1.