Jennifer,
You may not be doing anything wrong.
I always use the sander before attempting a join. Others might disagree with this approach and say that as long as your chopper or miter saw is in perfect tune, you should not have to use the sander and that it becomes a waste of time. This may be true, and I can't speak to that as I buy all my chops, and some suppliers have more accurate chops than others. So instead of finding out I have a bad chop at the point where I am trying to join the fourth corner, I believe it is worth the extra minute or so that it takes to sand all the miters.
Once the miters have been sanded, there is no guessing whether or not you will have a quality join.
-Mike.