Trying to build a polytych, but each time I duplicate the second image, the quality degrades massive

So I'm making a digital polytych of 6 images. First, some file info: 

Each image is 6240 PX x 3512 PX, RGB, 8-BIT, SCREEN sRGB. It was originally 72 PPI, then changed to 300 via going to Edit > image Size. I downsized these files to 1805 PX x 1122 PX to fit an overall file. 

The polytych file is set to: 11420 PX x 1320 PX, set to 8-bit, RGB, sRGB colour profile. 

To keep myself organized when I put it altogether, I put each image's individual file info (their adjustment layers etc.) Into a group. Here's my issue. 

I'll duplicate image one into the new document (poly file) and it'll look exactly like it's supposed to. But once I duplicate another image into the poly file, the second image is washed out, dull, and the blacks have a crazy amount of banding. 

I don't know what the hell I'm doing wrong. I tried duplicating different images as the first one to see if it was the file itself, but it isn't - the first image always works out fine, it's all the subsequent ones that keep degrading. Can anyone help?