Swapped screen, nothing appears on new screen once installed

I swapped out a relatives cracked screen for him and have done several of these before without any issue. This time however, I got it all done, powered on to test it and got a completely black screen. Could hear messages coming through, was able to see it when plugged into itunes, still nothing on the screen. I tried a hard reset, no luck. As I set it down, suddenly got a flash of the home screen for maybe 2 seconds, then back to blank. Tried moving the phone around again to try and re-create the situation where it would show the screen again, no luck. Powered it off blindly (so clearly the touch part of the screen is working) , unseated the display cables, cleaned them with a bit of rubbing alcohol thinking maybe it was oil transfer causing a bad connection, allowed to dry, reseated and retested. Same result..Even got another flash of the home screen at one point again, but went black really quickly. Tried replacing the old broken screen to test, no screen shows through that one either now. I am at a loss. Any ideas?

**UPDATE**

I've since brought it in to a repair shop, and they're running out of ideas on this one. They've swapped out the backlight filter with three different replacements and tried a whole new screen but still no image on the screen when turned on.

Late on the update on this one, but I ended up bringing it in to a repair shop. They couldn't find a solution either. We brought it to the Apple store and they said it was missing a screw. Funny thing is, I noted that when I first opened it up. Thought it was strange they would have missed a screw in the factory since this iPhone was only bought a few weeks before and I know it had not been opened and worked on in that time period. So all in all, I have no real idea what happened and ultimately no solution.