You've become desensitized to the flavorings in your juice. If you vape one or two exclusively, it's a good idea to try any other juices you might have, or may not have enjoyed. I let most of my juices steep when I'm not using them, and have found that when I start to become numb to a particular flavor, sometimes all it takes is switching to a contrasting flavor for a day or two. I vape primarily sweet, bakery types of juice, and have found that switching to tobacco or menthol flavored juice helps immensely. Again, you're going to want to vape the contrasting flavor for a day or two, but afterward your other juices should regain their flavor.