Here are some quotes from astrological texts by (1) Margaret E. Hone, (2) Jeff Mayo and (3) Myrna Lofthus, respectively:

(1) "Occasionally a planet is found to receive no aspects whatsoever. (The student should be wary of failing to notice the semi-square and the sesquiquadrate.) This does not seem to have been the object of much research. The idea is expressed here that such a planet will not be weak or ineffective (unless otherwise so adjudged), but, failing the interchanges with other planets, its principles, and especially its drive or urge when considered as an instinct or a force in the unconscious self, will not be properly integrated into the wholeness of the person. If a forceful planet, it will lack ways of using its drive. If a more receptive planet, it will at times be as if left out of the scheme altogether. For instance, a person with an unaspected Mercury will have moments of acting from sheer emotion, without the inclusion of any reasoning whatsoever."

(2) "This does not mean the planet is a weak actor in the chart. Usually it is indicative, according to the planet, sign and house involved, of characteristics or a feature in the life of the person which it is difficult, or perhaps not attempted, to integrate with the rest of the nature or life-pattern."

(3) "An unaspected planet is said to indicate an area (the planet, the sign, and the house) that you are trying to express but find it difficult to do, because the planet doesn't have the necessary "energy" to make it operate."

I would like to add, with respect to a very well known chart that has an "unaspected" Sun (at 28 degrees of Cancer and no other planets are over 20 degrees of any sign), that the late degrees (27 to 29 degrees) of any sign show up with inevitable frequency in the charts of people with whom the native has important relationships: mother, brother, girl friends, wife, bosses. Other people seem to be drawn into his life to "activate" the "unaspected" Sun. And transits seem to affect him more powerfully than they do the average person. For instance, when Uranus and then Neptune went over 28 degrees of Capricorn, opposing the Sun, he experienced acute manifestations of a series of "identity crises" both with respect to his domestic life and his career. This continued until Saturn in late Aries finished making a square to the Sun.

Any time a planet stands out, whether as "unaspected" or as a singleton (the only planet in a hemisphere, element, modality or orientation) or in any other way, it is important to question the client and elicit information and history from him or her concerning the matters in the life that are signified by that planet. Then, and only then, can you begin to understand how it "works" for that particular individual. Expect a great variety of responses from different individuals. There are no easy, simple "cook-book" formulas for interpreting "unaspected" planets - or anything else in the chart, for that matter.