HDMI Extenders with new Cisco boxes

I'm an AV installer who's having a huge issue. I wire many houses that have longer than recommended runs of HDMI to centralize in an AV closet. So I use Cat6 cabling to accomplish this, as I can get extenders that can go runs of 300ft.+. In the past 5 days, I've had 2 jobs where the boxes were all working at install. The next morning, none of them would work. No resetting or rebooting any of the boxes, extenders, or hdmi splitters solves the problem. This has just happened recently and I have no idea why, but it's going to be a huge issue going forward. Maybe even one where I don't recommend Cox to customers building new houses. Can someone please enlighten me on why this is happening? Apple TV plugged into the extender works flawless. Bluray works flawless. Cisco Cox Box...nothing. I can hear audio, through speakers, just no picture on the TV.