figured it out
I'm not sure what the root cause is, but I did figure out why the most recent albums purchases were no longer getting uploaded to google music.
When the Amazon MP3 Downloader would import the songs into iTunes, it wouldn't set the location correctly. Don't know if this was amazon's fault or iTunes fault, but it's what caused google music to not upload them.
Here's what iTunes said the file location was (right click->Get Info):
Here's what it should've been:
C:\Music\Amazon MP3\Erin Foley\Lower The Bar\13 - Dogs.mp3
I deleted the songs from iTunes, then re-imported them, and then google Music uploaded them as expected.
Not sure if this is the same problem any of you are seeing, but hopefully it helps.