It's a numbers game, the more you enter the more chance you have of winning. On average, a person wins 1 prize for around every 500 to 1000 entries.
I enter around 200 a day and sometimes don't win anything for months, sometimes several prizes a week. The easier it is to enter, the higher the number of contestants - sometimes hundreds of thousands for a single prize. The harder it is to enter, the less people will bother. I do very well on skill comps - story/poem writing etc, because most people just won't bother. I do miss the days of slogan writing. I won some wonderful prizes from them back in the day.