Why can I only see six of the ``Seven Sisters''?

Two major reasons: the number of stars isn't really seven (it's many more), and they all have different brightnesses, so that you see more or fewer of them depending on viewing conditions. The brightness of stars is measured on the magnitude scale, where the most prominent stars in the sky are around 1st magnitude, and the faintest you can see with your eye are about 6th. If the sky is hazy or you are near city lights, this faint limit may be 5th, 4th, or even lower.

The list below is in order of decreasing brightness. Note that there are nine different starnames, but two (Atlas and Pleione) are the parents of the seven sisters of Greek mythology. The stars 18, 24 & 26 Tauri are not named, though they have Flamsteed designations; stars lacking Flamsteed numbers are also nameless. The Pleiades star cluster contains hundreds of stars without mythological names because they are generally too faint to see without a telescope or binoculars.

        Name          Designation   Visual Magnitude
        Alcyone         25 Tau       2.90
        Atlas           27 Tau       3.62
        Electra         17 Tau       3.70
        Maia            20 Tau       3.87
        Merope          23 Tau       4.18
        Taygeta         19 Tau       4.30
        Pleione         28 Tau       5.09
        --             HD 23985      5.23
        Asterope 1+2   21+22 Tau     5.31 (combined)
        --             HD 23753      5.44
        Celæno          16 Tau       5.46
        --              18 Tau       5.64
        Asterope 1      21 Tau       5.80 (separate)
        --              33 Tau       6.05
        --             HD 23950      6.07
        --             HD 23923      6.17
        --             HD 24802      6.19
        --              24 Tau       6.29
        --             HD 24368      6.34
        Asterope 2      22 Tau       6.43 (separate)
        --              26 Tau       6.47
        --             HD 23712      6.49
Under very good conditions, all of these stars should be visible -- though 21 and 22 Tauri are close enough together (2.5 arcminutes) that the eye usually sees them as a single, brighter star (24 & 25 are even closer together, but 25 is so bright that 24 doesn't add much to it). Under more typical viewing conditions, the list will be truncated. The 0.79 magnitude gap in brightness between Taygeta and Pleione is larger than any other in the sequence. I think this makes it quite likely that the list will be truncated at Taygeta, giving six visible stars.

To illustrate this effect, here is a sequence of charts showing which stars within 2.5 degrees of Alcyone are visible for different limiting magnitudes.

[V <= 2.5 chart]
V <= 2.5; stars: 0
[V <= 3.0 chart]
V <= 3.0; stars: 1
[V <= 3.5 chart]
V <= 3.5; stars: 1
[V <= 4.0 chart]
V <= 4.0; stars: 4
[V <= 4.5 chart]
V <= 4.5; stars: 6
[V <= 5.0 chart]
V <= 5.0; stars: 6
[V <= 5.5 chart]
V <= 5.5; stars: 10/11
[V <= 6.0 chart]
V <= 6.0; stars: 12
[V <= 6.5 chart]
V <= 6.5; stars: 19/21
[V <= 7.0 chart]
V <= 7.0; stars: 29/31
[V <= 7.5 chart]
V <= 7.5; stars: 41/43
[V <= 8.0 chart]
V <= 8.0; stars: 63/65

In some cases, the number of visible stars depends on whether the 21+22 and 24+25 doubles can be separated. The higher-magnitude cases of 7.0, 7.5, and 8.0 are for illustration only; such faint stars cannot be detected with the naked eye. A chart labeling the stars for the V <= 6.5 case is available.

So, assuming the stars have not changed brightness significantly since ancient times, why then are there said to be seven of them? Well, some people have speculated that stellar variability may be the culprit; for one such example, see K. P. Hertzog, 1987 March, Quarterly Journal of the Royal Astronomical Society, v. 28, p. 27. But my own guess is simply that people like the number seven -- it's a nice low prime number which, unlike two, three, and five, does not generally manifest itself conspicuously in nature. As a result it seems ``mystical'', and people feel compelled to seek it out, say, in the number of visible planets (five) plus the sun and moon, or in the artificial construct of a seven-day week. The Hyades too were supposed to be seven in number, an abstraction which is even less plausible to the eye than that for the Pleiades. So much for numerology.

On a related note, it seems for the past few hundred years Andean farmers have used the appearance of the Pleiades as a seasonal forecasting guide for rainfall; cirrus clouds associated with the El Niño weather cycle obscure the fainter stars. For details, see B. S. Orlove, J. C. H. Chiang, & M. A. Crane, 2000 January 6, Nature, v. 403, p. 68.

Pleiades page