Roundouts too low, see it often. Sometimes people will start the roundout at the right time, but just be too slow in getting the aircraft level and bleeding off speed, then you end up 5 feet or lower above the runway in a level attitude and slight backpressure porpoises the nose, so people land flat. At the correct approach speed, how many "wingspans" should you be level above the runway (varies for different aircraft)? Then, how do you correct if you are a little fast or slow? Get quantitative and figure out some rules of thumb for your aircraft. There's a "magical" speed range that you have to get the aircraft into during landing that results in arresting the descent rate while raising the nose. It's not all that magical when you realize it's a matter of getting the aircraft to that speed range given the conditions such as airspeed, crosswind (drag), altitude, etc., but when you nail it, it hopefully makes more sense.