Whereas lots of ordinary workers, including many of these Walmart clerks of yours (who, you'd think, would at least hazily understand that their interests are directly opposed to the interests of the investing class), they simply blow you off when you try to discuss political issues with them - they don't want to hear it at all. Then later on they vote for a right-winger because that candidate promises them vague improvements in "public morality," or because the candidate dresses well, or because, due to big-budget TV ads like the Willie Horton one or the black hands one, they associate his opponent with those minority groups they so despise. How do you get through to these spoprts fans who refuse to listen to a word you say?
Like Carrol said, you must pick your targets. And what better way to pick your target then "fire for effect" e.g. try to discuss political issues with an individual or group and see what their response is; if positive, continue with discussion; if negative, don't bother them for the time being. People and their attitudes can and do change (at least by degrees); later, those who brushed you off might feel more open to discussing politics. As for why people whom you would think would hazily understand their class position don't want to talk politics at all, there have to be many different reasons per person and these reasons probably shift and change in importance (a sort of hegemony of the self) over time. The one reason which I think typifies most people who don't care to talk politics was mentioned by Noam Chomsky in The Manufacture of Consent (the documentary film): they come home from work, dead tired, turn on the TV, and just veg and shrug at what they see. Their own problems might not be important in the greater struggle over the means of production, but to them those problems are important enough that they just don't want any more on their plate. All this leaves out the feeling that they deserve some R&R from work and don't want to "work" more at some problem which, to them, involves someone else entirely (this last is, I suspect, ideology at work).
Todd