Why didn't Sandra try reasoning with people a bit more? (Spoilers)
Sandra's only argument is (implicitly) something like: it would be truly terrible for me if I lose my job, whereas your bonuses (+overtime possibilities) just make you a little better off, so vote for me as the on-balance best overall outcome.
When various colleagues either (implicitly) disagree with her assessment of what the best overall outcome is or indicate that they only care about themselves (and don't give two hoots about the overall situation), Sandra just caves and walks away.
Now, maybe this just speaks to Sandra's depression, but I would have thought that Sandra would have back-up lines of reasoning available (hell, even offer to pay the ninth and tenth votes half their bonus if that's what it takes!).
That is, I expected Sandra to at least try some explicitly moral reasoning focussing on reciprocity, solidarity, etc.. E.g., 'Don't you get it? If I get bumped off in this way, then if Jean-Marc next decides he doesn't like *you* for some reason (perhaps you're off sick for a little while) then you could be in the same pickle: everyone gets offered a bonus if *you* get fired. You wouldn't like that; you'd know that that your colleagues were being lured into doing something shameful; and you know that that's how it is with me now. We have to stick together...'
Moreover, Sandra never tries any explicitly Christian or quasi-marxist/workers v. bosses lines of reasoning (the former is brought up by one of her colleagues near the end).
Possible thought: perhaps part of the discouraging picture the Dardennes want to paint is that of a modern Belgium/Europe largely devoid of moral/normative vocabulary. Sandra is depressed but she's also just got no way of offering reasons to her co-workers, rather she can only talk about herself and let them talk about themselves. Sandra's encouraging 'breakthrough' at the end is that she at least enacts a moral standpoint, one that automatically gives her self-respect through having exhibited respect for others dignity.