Nothing on display after battery replacement?

So I have been doing iPhone repairs for a couple years now. Had one come in today, just for a battery replacement—but with the new battery I'm getting no display at all. Even tried multiple batteries, multiple screens, and even new charging port.

Any ideas as to why I'm not getting a display? The display will flash like the phone is on for about 1 second, then goes back to black. Has done it multiple times. Just cant get it to keep the display on.

Any suggestions will be much appreciated. You can email me at v . r o t o l o @ c e l l u l a r s o u r c e . n e t (no spaces) thanks in advance.