In a democracy as such, the total feedback control from the governed to the ruling is in the order of a few bits per year at most. Certainly, any process to satisfy millions of people with diverse preferences requires much more information than that. What is worse, the ruling class itself largely gets to define what those bits encode. And what damns the whole system, the individual incentives are for the ruled to not acquire such costly useless (to powerless them) information, and for the rulers to spread propaganda that will extend their power.
In other words, the powerful are pretty much out of control of the citizens. Certainly, a few bits per year of information might sometimes be better than none at all. But not for long, since the meaning of those bits is soon to be controlled by some variant of a two party system. The knobs controlled by those few bits will never allow to change the one thing that matters mosts: the irresistible growth of the power exerted over you, the fact that whoever is likely to be elected is a power hungry bastard backed by an organized predation system. Preserving and extending the power of politicians and bureaucrats upon citizens is a bi-partisan issue.
Inasmuch as some democratic societies work and others don't, it isn't due to democracy as such, but to other institutions completely independent of democracy, disconnected from it, and actually slowly but surely corrupted by it and destroyed by it as well as by any other form of political power: individual rights (as opposed to collective claims), common law (as opposed to statute), the rule of law (as opposed to the arbitrary power of politicians and bureaucrats), a culture of honesty (as opposed to having to weasel around imposed regulations), widespread self-reliance (as opposed to a sense of entitlement), and other personal moral values (as opposed to compulsory submission to "moral" rules edicted by others). These are the institutions of a market society, one where each one earns his living out of mutually voluntary cooperation.