Why can't youeat more than one meal a Day

Maybe what you eat for lunch contains large amouts of fibre, and as you know fibre 'grows' in your stomach, therefore fills you up for longer (for me its very long). If you think this is affecting your concentration in school etc, you should cut down on fibre and switch to other foods. Try get down the gym too, as it should increase your appetite ( but you will only gain healthy muscle if you keep exercising )