First obvious question is, do you have WhisperNet on? Please don't be offended. . . can I get an "aye" from those of us who thought it was on and it wasn't? I'll start: "Aye".
Now that that's out of the way, the next thing to try is to click 'sync and look for new items'.
If that doesn't work, I've had good luck with re-setting the device. It clears out any behind the scenes clutter and when it's rebooted the new content usually shows up. I think you can do a soft reset on either Kindle via a main menu option, or using <alt><shift><R>.
I've also found that directly "buying" the Amazon Kindlebook from my Kindle results in immediate downloads, whereas "buying" through my computer's Amazon.com connection is sometimes delayed considerably - and then, when checking my Kindle account, it says it's downloaded, when it has not yet happened.
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