In concurrence with the above post, the conch is mainly a symbol of control. Unfortunately, Piggy does not understand this symbolism at the beginning of their being stranded on an island. For, when he tells Ralph about observing some others once use a conch and get attention, he seems to attach some power to the conch itself. It is as though the conch is a talisman to Piggy, and later to Ralph, as well.
Then, in the later chapters when, in an attempt to arrest the anarchy of Jack and the hunters, Ralph calls a meeting and holds the conch, he is very nonplussed when the boys do not react properly at the sight of the conch. The "problem" with the conch, then, is that it has significance only when the boys attach significance to it. This difference between true meaning and symbolic meaning is illustrated when Jack declares that he can build a fire, and all the boys but Ralph and Piggy run off to build this fire. This action of the boys indicates the dilemma of civilization: rules must be agreed upon and honored with symbols recognized as representative of the civilization. Otherwise, there is anarchy.
I do not really think that the conch itself becomes a problem. Instead, I would say that the conch is just a symbol of the real problem that actually does exist. The conch is just a symbol of (and a pawn in) the struggle between Jack and Ralph.
The whole story, to me, is the story of the struggle between civilization and the good side of human nature, represented by Ralph, and savagery and the evil side of human nature, represented by Jack. During this struggle, the two sides fight over the conch at times, but the conch is not really what they want -- what they want is control.