Google not indexing pages - is site structure to blame?

Hi guys,

I would really appreciate some help on this as it is driving me crazy! We have a medium-sized website with over 1800 pages, most of which are not being indexed by Google - not good. Firstly, these are not pages that we've recently 'thrown up', most have been live for over a year.

The website is Download Free Stock Footage | Free Stock Video and HD video clips - Videvo.net - we offer free stock footage for any project. The clip download pages (which you'll see if you click on a clip thumbnail under browse page) are not being indexed by Google. There are almost 1800 of these, and it would provide a big boost in traffic if they were. My question is, what is causing Google not to index?

My initial thoughts were down to site structure, or poor coding, but the code doesn't look terrible to me (I am not a programmer so not 100% sure on this) - another thing I considered is that there is not enough readable content on the pages for Google to consider indexing it - maybe it doesn't think it's worth it due to lack of readable content.

Any help really would be hugely appreciated. Also if there are any of you out there that are interested, we are considering paying for a full site audit so we can optimise pages and structure for Google. Please pm me if interested.

Thanks a lot