Several explanations have been offered to explain that seventh star. Perhaps one star has gone dim, or perhaps interstellar dust swirling around the group and the dust has thickened around one star and has obscured it from view. Long exposure photos support this idea, but a dust cloud would take more than a few thousand years to change the brightness of a star. Also, a few thousand years would also not be enough time for such young stars to dim for any other reason that we know of. Another possible explanation is that the seventh star may be an erratic variable.
Variable stars are stars that vary in brightness over a period of time.
They cycle between peaks, going from brightest to dimmest. Only a few naked eye stars have a large enough variance to actually be seen, however. But there are some variables that have an erratic period, and the variance is much greater in both time and magnitude. Pleiades you see, they are a pretty sight. If you go out to look for the Pleiades, they are very easy to find, and they will be with us all winter.