Not remembering your dreams could just be down to the fact you dont take them seriously enough or there often embaressing or weird and you feel them unnecessary to recal from.
If you lead a busy life or systematic routine they fall by the way side and some are just to differcult to piece together random images, unknown faces,places ect.
it says nightmares are often forgotten in this book but i dissagree i remember nightmares far more vividly but i watch alot of movies and weird films lol