Hi Claire,
When we see screen problems like this, generally there are three places we look for the source of the problem.
The first is the connection between the screen and the motherboard. On Samsung screens there's only one connector, so that's the first thing you want to check.
Open the phone back up and follow the guide as needed to gain access to the display connector. Unplug it and check both sides, the part on the flex cable and the part on the motherboard, for any kind of bent or broken pins. Bent pins can sometimes be straightened with some fine tip tweezers, but broken pins are more problematic.
Assuming you don't see anything wrong there, take some 90% or higher concentration isopropyl (rubbing) alcohol and clean both sides of the connector. Let it dry for a minute then plug it back in and see if there's any difference in the display.
If it's not the connector, then it's possible you simply got a defective screen; despite their best efforts it happens sometimes. The good news is that iFixit's screens have a lifetime warranty and their customer service department has been excellent to work with when I've dealt with them. Best way to reach them is to email their Store personnel at support@ifixit.com.
The last possibility is an issue with the motherboard which is outside the scope of what us mere mortals can do on our phones. I tend to discount this as being very likely due to the fact that the two screens you've had both react completely differently; if it was a motherboard issue they'd be doing the same thing.
So yeah, check that connector and if it's still not working I'd suggest contacting the iFixit Store people and talk to them about the situation. If you plug the old screen back in (no need to actually install it) and can verify it's still working like it did, then that would tend to argue for an issue with the replacement screen which will almost certainly be covered under warranty.
Good luck and let us know what you find!
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