Hi,
Do the snapshot operations buttons (create, delete, restore) appear but grayed out or do they not appear at all?
If they don't appear at all then this seems like a permission issue. Can you please assure that the logged in user is assigned with either UserVmManager or SuperUser roles for those VMs on the relevant VM level or VM's data center/cluster level?
It might be that importing those VMs, removed those permissions for the VMs.
If the snapshot operations buttons appear but grayed out then this implies that the disk images or snapshots are locked due to a previous VM operation.
Thanks,
Sharon