Back, Home and Task Nav buttons don't work in portrait.

Hi there. I've recently finished completing the screen replacement instructions (as found on this site) on the Experia M2 but I have a problem still. When the device is powered up and held normally in the portrait position (rather than landscape) the Back, Home and Task Navigation buttons at the bottom of the screen are unresponsive. However, if I rotate the screen through 90 degrees and hold it in the landscape position they work fine. Does anyone know why this would happen and how I can resolve it please.

Thank you in advance.