Are the networks identical on both hosts? VSS or VDS? Any hardware affinity? Any DRS anti affinity rules which could interfere/cause to be moved back? Anything in the VMware.log or vpxd.log? Maybe worth restarting the management services if strange things are still happening.
After further inspection it seems all of the VMs that failed to migrate were on LUN01, and Host B is having intermittent issues 'seeing' LUN01. In the past I have done some action at the command line to get the signature squared away but apparently that fix did not persist.
Other 20 LUNs are fine, not sure why this one causes an issue, I bought a Compellent SAN from Dell, direct-connected to the hosts on SAS hopefully Compellent doesn't end up being a mistake.