Well, sorry, I don't know either if it is a hardware issue ( may need to send the phone to Samsung or replace the screen yourself )..... or if it is a software issue, of some sort, might be installed program which doesn't work or something else like that. Since you already did a factory reset, the only thing I can suggest is to root your phone and see if the problem is still there after rooting and installing a custom ROM ! If so, it's probably a hardware issue..... Only one way to find out !!!
Sorry can't help you better with that......