If you're not remembering your dreams, it could be many different things.
Sometimes, dream recall can suffer when you're really stressed. If so, you might try meditation or relaxation. It helped me a lot.
You could also try waking yourself during a period of your sleep when you'd be dreaming- this is a few hours after you fall asleep. Set an alarm clock or something, and journal anything you remember right away.
If you don't want to do that, then as soon as you wake up, you should lie as still as you can and think about nothing else except what you just dreamed about. Don't even journal yet, just replay the dream in your mind and lie in bed, just as you were when you woke up. If you can remember any tiny detail of your dream at all, even if it's just an emotion or feeling, write it down! It will help in the long run.
I hope this helps!