Spark Download problems

I created about a 2 minute Spark video and all was going well. Yesterday I finished it off and proceeded to download it, and it isn't downloading now. It seems to get stuck at the beginning of the download and it either just stays in a frozen download mode or eventually gives me a generic download error. I'm not sure what to do and need to get this project finished up....help! Any suggestions on what to do?  The problem seemed to start when I replaced a photo on one of the Spark slides (it had been to large to load so I recreated a new photo file that was smaller, replaced it and everything seemed fine...then the download problems started. Also, the preview of the video runs just perfectly. So I'm stumped! TIA for any suggestions.