Originally Posted by televisor

Are you a Canadian citizen? I believe they get special treatment at the border?

CBP do claim to stamp every visitor (with the probable exception of Canadians), but clearly they don't always do so in practice. That said I've always been given stamps, even the one time I've used a GE kiosk (where I got an X). There's certainly some disagreement on the internet about how important the stamp is, but it's certainly useful if you ever need a proof of lawful admittance. (That said, SSA and the DMVs all require your printed I-94 and they don't care about stamps. I'm not sure what other agencies might want to see, especially in the case of inland border checkpoints.)