completely dead after screen replacement

I've replaced over a hundred 4th gen digi/lcds and every so often I get what I call an "epic fail".

I'm very careful not to tear any ribbon cables and have replaced numerous ones that have needed it. It seems Ive done everything right but when I go to turn it on, nothing. Plug it into charger, nothing. Try to sync with Itunes, nothing. Hold the power and home buttons for an hour (lol) nothing. Ive had 3 in the past week with this same problem. Please help!v I can't afford to replace these ipods and cant figure out where things went wrong.