You have to be slightly agressive. Always keep your reminders polite. I had problems with one professor and if I wouldn've played a passive role I would still be waiting.
I both emailed my professor and also dropped by her office. It's not that they don't want to write it; rather its that they are both very busy and and when they do find free time they find other things to do. Remember, though this process means all the world to you, it's only a forethought in many of the recommender's mind.
What worked for me was a a third gentle reminder saying that my secondary materials were on hold at various universities until they recieved the LOR. And although I wholly understood how busy she was, that it would mean alot if she were to have the letter ready within X amount of days.