I'm guessing that on average I taste about 10 new wines a week to evaluate whether I'm going to stock them or not. Most of the time the tasting are pretty straightforward. There are winners and losers. Some days though everything tastes bad, other days everything tastes great. I don't actually trust when either of these happen. Usually it means something is off with my palate that day. I learned that buying something on an "everything is awesome!" day will lead to disappointment later at least some of the time. If everything tastes awful on a particular day it's worth my while to reevaluate those wines on a different day.
Sometimes the way you appreciate a wine (or don't as the case may be) is circumstantial. What are you eating it with? Are you with friends and having a good time? Are you having a bad day? All of these things play a role. A common thing that happens around here: someone has a great bottle of wine with dinner at a restaurant, they come on and order a case, they take the wine home and are very excited. The next time I see them they will tell me, "The wine is good, but I remember it being different when I had it at the restaurant." The wine is the same, but the context has changed.