iPhone 4 doesnt load

Hey,

im realy into repairing iphones, but now i got a thing that i doesnt understand. i have replaced a whole iphone 4s case to a new one and everything was replaced 1:1 . but after loading it a but now the iphone doesnt load anymore. i tested 2 new flex load cables so it cant be the cable.. what could be the failure then?

thank you