LCD turns on then off after replacement

I got a third party display and front glass + digitizer for replacement. After following all the steps in the guide, I went to test both while the iPad was still open.

At first, the LCD turned on as normal, the home button worked, but the digitizer gave me nothing.

Then, after fumbling a bit, trying reconnecting both, the LCD started turning on then immediately off every time I rebooted the iPad holding both the home and lock buttons for a few seconds.

The screen only got to showing the Apple logo for a bit, then going black, including the backlight.

The iPad stayed on as it made the charging sound, and got picked up in iTunes.

I thought I had killed the display, but no, the old one also did the same thing when I tested it.

So what could be the issue here? thanks