I had a couple of problems on each of 2 ATV jailbreaks. But over the last year I have found that most of the problems were on my end. Make sure your USD is plugged in all the way when going to DFU. Make sure no other devices are connected to iTunes or computer. My iPhone uses wireless sync so I even turned it off. I don’t even multitask and do other computer or wireless activity on my local network. Do plug or unplug anything until it tells you to. And make sure when it says to disconnect from USB/DFU and connect to TV that you do it quickly, connect the power supply first, then connect the hdmi.
When I was having problems, in the past, someone advised that the iTunes Application can not be buried in another folder inside the Applications folder. So I now have it just in the Applications folder.
I find that I have to follow the directions precisely, word for word. It is very fussy with any deviation. When I do that it works everytime.