black screen after screen replacement

After replacing the LCD/digitizer on an iPod 4th Gen it will not turn on. I have tried holding down the home button and power button, no luck. I have left it for 24 hours and then plugged into a wall charger, still no luck. I have tried shorting the positive battery terminal to drain the battery, no luck. Double checked all the cables, ribbons, tried a different screen, even tried the broken screen that was working when I took it apart. All with the same result, black screen, computer does not see it and there are no sounds.

I have successfully repaired many iPod 4th Gen but the last 3 in a row have given me this same problem.

Any suggestions and help would be greatly appreciated.