Two questions I've found while digging around.

Q:Why can I control X but not Y?
A: Dream control isn't uniform. Some things are easy to do because you convince yourself that they are easy, and others are hard because you believe them to be so. Starting out with things you know that will work is a simple way of eliminating doubt and giving you confidence. And of course, if something doesn't work at first, you can always try it again later!

Q:Why did I suddenly stop having lucid dreams?
A: That is called a dry-spell. Sometimes, for several reasons (like stress, changing sleeping schedules, eating habits, etc) a person cannot achieve lucid for some time (it can be days, it can be more than a week). This is not reason to be discouraged, and it's solved by having patience, letting your mind go off the subject for a bit, and keep practicing. Eventually the lucid dreams will show up again.