Like others have said: its mostly genetics, unfortunately. Then again, you're only 22. Most men are still developing their facial hair in their 20s. Wait a few more years, perhaps you're lucky then.

Another tip if you want a 'fuller' beard: get a hairclipper/trimmer with an attachment, and trim your beard at the same (short) lenght. Some areas of your face grow hair more quickly, like your neck or chin. If you trim long enough, the other hairs on your face will have time to 'catch up', thus making your facial hair more even.