Dead Pixel on replacement display

Hi, ive just had a replacement display fitted to my 15" rMBP which had a small dust mark or chip in the screen, but apple said it was a dead pixel. Now on this replacement screen i have allready noticed a very small dig/ dead pixel in the middle of the screen. Does anyone know why this keeps happening, I cant afford to get the screen replaced again due to needing the machine for uni work. I do still have a year and half left of apple care which i used to replace the screen last week, so i can always get it replaced next year before it expires.
Thoughts?