iTunes won't sync purchased music to iPhone

I'm trying to update the music on my iPhone however my iTunes will only sync music that hasn't been purchased.

That is, I have 400 or so albums that were ripped and not purchased from the iTunes Store that sync across fine. However the 100 or so albums I've purchased from the iTunes Store refuse to sync across even though they've downloaded and are waiting, as hard files in my iTunes library.

This would be annoying but tolerable if I was able to just turn on show music in the cloud on my iPhone (which I've done and the albums appear) and download them that way, but none of the tracks will download for me when I select download. So I can't listen to any of my purchased music on my iPhone.

Please help, this is very very frustrating.