Try cleaning the zebra strip (silicone strip) and PCB contacts with isopropyl alcohol and a cotton swab. Leave no residue. Don't touch the glass of the LCD where the strip makes contact as the very thin gold contacts (which you can barely see if you hold it up to the light just right) are very delicate. You could gently clean the LCD contacts with a swab soaked in alcohol, but you have to be very careful, so it it best to try this first: Reassemble the unit and power it up. If the display still has missing pixels, squeeze the LCD to the PCB gently to see if that fixes it. If so, you'll need to retension the LCD holder.