There are a couple of reasons why there might be gaps in ourdownloadable eBook/audio bookcollection:
Not all books are contracted to be published in all formats. There are varying reasons for this, but one example is that the authors may change publishers and contracts while in the process of writing a series. If the new/different contract does not cover digital publication rights the item will not be available in electronic format.
Not all publishers work with OverDrive. The library, through OverDrive, looks to select books for their digital collection and are only able to select from available items.