Mason Dixon KAL

I'm not a new knitter, I've done a Dale of Norway sweater, a cabled vest, felted bags and flowers, socks and mittens.... why am i having so much trouble with the mitered square? I've ripped it out 4 times. when i get to the end, i'm a few stitches short for finishing according to the instructions and the "square" seems a bit off?! help!