2 iPhone 5s. Getting only light no-no apple sign on 1

iPhone 5s

here are two exact Verizon iPhone 5 phones a1429.

the one on the left need to be working, the one on right is parts. I'm using the same digitizer (new) to test both.

the one on right when connecting new digitizer, for testing, boots all the way to phone.

the one on the left (important one) used to boot to the Apple sign, but now only lights up the background (still black just lighter). I did actually have this left one booting initially but stopped. what do I do wrong to the left one?

I've swapped batteries, that doesn't change anything-right one still works.

is there something you see on the left one to where it's not fully booting or lighting the LCD anymore?

I'm assuming, if needed, I can swap parts, but can I swap logic boards?