Hi Rubystars, I just took a semester of biopsychology and we did a chapter on hunger, appetite and saiety, so this is an intresting question to me. I'd have to find my textbook and look it up to give a technical answer, but basically there are gauges in the body that measure the available nutrients in the blood. When levels are sufficient they trigger a release of hormones that produce the sense of satiety. You can be full, and still not have enough nutrients in your blood to trigger the hormones that tell you to stop eating.
It is probably a good idea to keep track of what you are eating that is causing this and see if you can maybe make it more balanced in terms of carb, protein and fat. I know I've experienced this, a lot of times it happens with salads. They are bulky and make me feel full, but I'm still hungry. Usually a few almonds will make me feel better.