Panasonic Viera TV HDMI handshake issue?!?!

I have a couple of Himedia Networked Digital Media Players, both of which connect via HDMI to my Hisense TVs. However, when I try to connect them to my dad's 2007 Panasonic Viera TV I just get a blank screen. I can connect them with AV cables, but that obviously isn't ideal and I'd prefer an HDMI connection. Even more strange is that when I tried connecting with component RGB cable I get the same blank screen. Does anyone have any idea what the problem could be and how to solve it?

Thanks in advance.