Onkyo 608 Display on HDMI 1 only (Other ports work)

[sl4y3r]

I can't seem to connect my PCs (i have 2) to any other port apart from HDMI 1. I know my other ports are working because I've connected my MackBook and friend's PS4 to them.

Any suggestions?

One is an Intel NUC7
One is a Desktop PC with a Nvidia GTX 1080Ti

Both of these are 2017-18 devices and both work fine only on HDMI port 1, only.

Please help!