I can't download purchased audiobook onto iPhone

I can find my purchased audiobook on my iPad and download (from iTunes) but I can't seem to find it on my iPhone 4 to download the book. I have tried going through purchases, audiobooks, etc. is not working..I'd like to ba able to listen to the book on my iPhone also. Shouldn't this book be in ICloud? I have backed up everything at least once using ICloud..