cannot download videos source file cannot be read

I have a MacBook Pro 2.9GHz Intel Core 17. 8GB memory, 577.7 GB free of 676.63 GB Macintosh HD, 843.5 MB Free of 72.3 GB, Version 10.10.5. My problem is when I go to download a video from a purchase I have made it says it cannot be saved because the source file couldn't be read or it will tel me to contact the server administrator. What do I need to do in order to successfully download my products?