When they lose their flavor do the rest of your juices still taste good? If not, or if they're muted, you may need to drink more water. Another possibility is taste buds becoming re-accumulated, if you recently stopped smoking (past 3 or 4 months), they may go in and out for certain flavors (or all together), but water will help with this too...some.
The last possability is something along the lines of eating the same food all day, you will eventually loose your taste for that food. Personally I would sooner go for the first few possibilities before looking into this.
No matter what the case, I suggest drinking more water than normal, and expect your tastes to change a bit in the comming month(s). When I first started vaping certain flavors were just vibrant, and they now seem bland to me. Other flavors were favorites, and now I don't care for them. My current favorite doesn't even taste the same to me as it did a month ago, but it is still tasty as hell, and still my favorite.