VM Compatibility ESXi 5.1 and later (VM version 9) trouble with network

I'm having and ESXi host with 10 VM's on it, all connected to the same portgroup on the virtual switch. In order to enable VHV I created the last to machines with compatibility ESXi 5.1 and later (VM version 9), where all other machines have ESXi 4.x and later (VM version 7).

Now I'm having the issue the 2 machines with version 9 can only talk between them, and the 7 version can only talk to each other. It is not possible to ping a version 7 vm from a version 9 vm and vice versa.

Any idea,a re there any known issues, something I did wrong? According to what I discovered so far there seemd to be a major change in networking between those versions.

Any help verrrrrry welcomed!

Thanks, Philippe