If the call is relayed, you cannot make HD video calls.

To check whether your Skype-to-Skype call is being relayed:

1. Sign in to Skype.
2. In the menu bar, select Skype > Preferences…
3. In the Preferences panel, click Advanced and check Display technical call information.
4. Make a Skype-to-Skype call.
5. In the menu bar, click Window > Technical Call Info.
6. A box with technical information related to the call will be displayed. If the call is relayed, the word next to SessionOut and SessionIn will be RELAY_UDP or RELAY_TCP.