Downloadable Products - Links just load the product page and don't download the file!

Hello,

I am trying to sell downloadable products but when the download link is clicked, the file is nowhere to be seen. It just loads the original product page (albeit with a /?orderid=75a0ebf84d2f at the end of the url)

Check Points:
The file I'm offering for sale is on the same site / server as my WP install but was uploaded via FTP and not the upload form on the edit product page.
The pasted the url works: it is definitely not broken.
The file is a .mp4 video file and is about 100mb in size, would the size make any difference?

The Marketpress settings are assigned to have the link expire after three downloads. Although attempting to download 10 or so times, the link hasn't expired (despite it taking me to the product page every time)

Please advise

Thanks for your help in advance