Where are you located in the world? How old is older? What types of files? Are all these purchases on the same Apple ID?
See HT5085 - iTunes in the Cloud, iTunes Match, & iTunes Radio Availability > iTunes in the Cloud availability by country to see what should be available automatically. Where files are not available for redownload this is typically because the rights holders haven't yet reached agreements with Apple (which is why there are different rules in different regions, or for different content such as audiobooks) or because specific content has been withdrawn by a rights holder, which can happen at any time.
Note that any request to requeue past content that isn't available automatically would need to go via the "report a problem" links in your account history or an iTunes Store support contact form, not through a telephone support line.