It happens all the time to me too. I like my stuff crazy sharp, all the time. When you get the edge that fine IMHO minute degradation of the edge occurs from the usual suspects, primarily water. It happens to all types of steel eventually. You also have to take into account that if you are testing it by touch that your skin or nails are not the same as they were a month ago. And you may be leaving minute amounts of acidic oils from your skin. A slight touch up with a glass, ceramic rod or strop will bring it right back. I live in a very humid environment < 1/2 mile from salt water which is also a factor. YMMV depending on your ambient environment. It does not seem to matter whether it is carbon or stainless steels, although I do take extra care of steels more prone to staining or rusting.