It seems that you need to resume from the very same person (i.e. the one with the same IP) in order to secure proper resume. If you resume from someone else, there's a high possibility of starting up a new download, or at the best, resuming, but starting from scratch again.

I would take a note of the exact file name and the IP address while downloading, and try to find the very same IP when attempting resume (i.e. I'll be "repeat searching" for quite a long time to find the IP). I would be very happy to hear what others are doing with this issue.

I don't know much about programming, but this might be something worth tackling in order to improve the software.

Best of luck.