I have sound, through the optical cable, but no picture

[Lew]

I have sky, Firestick, Chromecast all feeding into an HDMI Switch. There is one output from the switch to the Sky Divialet sound box and then an HDMI from there to the TV. There is an optical cable from the sky box directly into the soundbox.

I have sound, but no picture. 

I've bypassed the switch and taken the cable direct from sky box to sound box but this doesn't work. 

please help.