Unable to Add Host to vDS 5.1

Hi to all,

I've two infrastructure:

     1) vCenter 5.1.0 build 880146 with 6 host 5.1 upd1

     2) vcenter 5.5 upd 1

I want to movehosts fromvc 5.1 to new vc 5.5

I've export vDS 5.1 from vcenter 5.1 to vcenter 5.5

I've moved and added 4 Hosts without problem to new vc 5.5 and vDS

Now when I try to add the last 2 hosts to vDS I received this error:

vDS operation failed on host esx06.sf.local,  got (vim.fault.ResourceInUse) exception

vDS operation failed on host esx07.sf.local,  got (vim.fault.ResourceInUse) exception

How can I move the last hosts?

Thanks to all for any suggestions.