| This could be because you are going through a proxy server and the proxy does not support resuming. If your proxy doesn't support resume, don't use it unless absolutely necessary, it will save you a lot of time in the long run.|
If you can resume some downloads but not others, then the problem is not your proxy server. It is the server that you are getting the file from that does not support resuming. More and more web servers are supporting file resume, so this situation should be rare. BulletProof Go!Zilla will resume downloads from BOTH FTP and HTTP sites that support resume.