I was doing some storage migrations and in one of the clusters Storage vMotions failed with a message: Failed to resume destination VM: msg.vmk.status.VMK_MEM_ADMIT_FAILED.
After closer look I found that all those VMs that failed had a big memory reservations (192GB and 256GB). The ESXi 5.5 host only had 512GB of RAM. I will not go the topic why those reservations were set, but to successfully Storage vMotion those VMs I temporarily removed those reservations. After that Storage vMotion was successful. My guess is that VMWare check during Storage vMotion does the destination host (even though during Storage vMotion destination host was the same as source) has enough resources for this VM and if it does not it will fail the migration.