Hi Michael
It isn't unusual for taste to change for a variety of reasons but as Eskie and tazzle suggest if it isn't a change related to some change in the actual
vaping method (new vs old coil, different temps/power, airflow settings etc) then it is one of two things. Either the juice was still steeping and actually changed its flavor over time or your taste impression changed.
The latter is not uncommon. After vaping the same juice over and over the taste can seem less than it was. Try different juices for a while and then come back to your favorite. My taste attention span is very short so I almost always have at least one other vape handy at all times.