I think that a variety of factors can lead to this.
The most obvious one is the listeners mindset. Sometimes I sit down to listen and discover that I listening isn't the right activity for the moment.
Other times I'm am convinced that my system simply isn't sounding as good as it sometimes does. I wonder about the power grid impacting the sound as it seems that during the day is when it routinely doesn't sound great. This was actually what caused me to play with power cords a bit. If money were not a factor I'd look for something like a PS Audio power plant just to see if I felt like my system sounded more consistent.