I have ran into issues like this in the past. We used to have comcast and downloads would take forever to download and probably never reach 100%. Try resetting your router. If that doesn't work than call your isp. I ultimately determined the cause of my issues was because the copper line running to our house was installed back in the late 1990s. We changed ISPs to FIOS and they had to install there own line into our house. Now we get the 50/50 download speed we pay for, sometimes more!
Edit: Also, check to see if there is any packet loss on your internet connection. That could also be the cause of your downloads not finishing.