Hi,
This sounds similar to the issue in this thread here: AND again no full sync = second time
The gist is that Amazon is throwing errors that stop the recursive download process and require it to be kicked off again. We plan to adjust behavior to try to plow through more errors that are considered transient in nature. We currently do for some exceptions, but not all.
The larger the amount of data to be downloaded, the more likely you are to hit some sort of error that stops the processing. Restarting the sync will continue on, as you have seen.
How much data are you attempting to download?