We have already discussed how to intentionally do a "partial" download. But maybe it's not> on purpose. You may have sent a large number of rolls to the lab, and a full download will run into the gigabytes. Sometimes, during such lengthy sessions, Bad Things Happen. You can have a power failure, loss of internet, the lab server might go down--there are plenty of possibilities.
The time already spent has not been wasted. If you leave the partial download where it is in the destination folder, FTProofsheet can pick up where you left off.
When the issue is corrected, restart the download. FTProofsheet will check the files that have already been downloaded, and completes any that were interrupted midway. The session will then continue normally.