It could be you got a digitizer that has issues. I got one myself once, had to put pressure on top left hand corner to get bottom left hand side to register. In the end I sent it back for a replacement, and the replacement worked fine. If you go through the other steps posted above and check all the flex cables/connectors out and you still have problems, my money is on a faulty digitizer - it does happen occasionally.
HTC One X running Viper X 4.1.1
Samsung Galaxy S2 i9100 running Fusion V1.5 S5 theme KitKat 4.4.2