My kindle has stopped automatically syncing to my recent purchases. I've had to go onto my amazon account and tell it to download my new books. I've got a good internet connection and it's only just started doing this. I haven't changed anything recently on my settings or passwords.
I experienced the same situation. I went to my account and downloaded those books (all of your Kindle purchases are listed there). I installed Kindle on my Mac and my phone in the last week so I can use them alternatively. I bought 2 books since and they were downloaded so I don't really know what's going on.