Hello, and welcome to the forum!
Sorry to tell you, but I think this is just the nature of the beast. I also have a bottle of MI, and some days it just "Wow!"s the heck out of me and other days I either fail to notice it much and/or it has me second-guessing my purchase. I'm running just about 50/50 on my observations.
I think it is equal parts the scent itself and differences in ourselves. Our sense of smell certainly is not the same day in and day out, plus the ambient air characteristics (temp, humidity, etc.) affect both the scent and our sense of smell. So it makes sense to me that the same scent smells differently from day to day. This is especially pronounced with scents that have lots of fresh citrus or fruity elements, which by their nature are quite volatile. This changling nature of scents may also be most prominent when the weather is changing or varying a lot, like during the changing of the seasons.
Despite these observations, I do consider MI an excellent fragrance and I'm glad to have it in my collection.