Try restarting Steam. It happened with me a few times. I'd put an 8 gb file for download at night and when I check back 9 hours later, it showed only 4gb downloaded. I have a 4mb connection so obviously, that was bs!! I exited Steam and restarted it after about 30 seconds and the file showed as completed!
^ This. Steam is currently having massive sales, and the amount of data their servers currently has to displace is immense. They're struggling to keep up, but just give it time for people's monies to run out and it should get better.