Once possible reason is that you are no longer quite so motivated to lucid dream. After doing it a few times, lucid dreaming no longer seems quite so incredible - maby even just subconsciously.

If that's the case, its not too hard to fix. Just remind yourself that in lucid dreams, anything is possible. Whether its flying to the moon in your bathtub, exploring hidden treasures, or finally talking to your crush, you can do it. There's also the unique feeling of the lucid dream: being in a world that feels completely real, yet know that you are completely safe from harm in anything you do.