After upgrading to Virtual Center 2.5 Update 2 you may notice that guest VMs no longer automatically VMotion off the host when you attempt to put it in Maintenance Mode.
This is a new behavior introduced in Update 2.
No virtual machine migrations will be recommended (or performed, in fully automated mode) off of a host entering maintenance or standby mode, if the VMware HA failover level would be violated after the host enters the requested mode. This restriction applies whether strict HA admission control is enabled or not.
The solution is to either disable HA on the cluster, or migrate the machines manually. Once all machines are off the host, it will go into maintenance mode.
The release notes mention this here: https://www.vmware.com/support/vi3/doc/vi3_esx35u2_vc25u2_rel_notes.html#knownhaissues
Think you have enough HA capacity that this is not your problem? Try un-configuring HA on your cluster and test. If it goes into Maintenance Mode after this, then this is indeed it.
My understanding is that this will be corrected in a future release but don’t hold me to it.