I am experiencing similar problems with b5, not previously.
1. Download stalls due to missing pieces, assemble incompletes everything goes grey but nothing moves, run quickpar and some of the parts are missing (no sign of them anywhere).Restart Newsbin, the missing parts are assessed as missing and downloaded again, repeat until lucky.
2. Downloads in progress, Restart Newsbin, some/all works in progress lost.
Possible resolution to 1-2;
I moved the data directory to a share (autoload and download directory already there) which is not virus checked (Avast WHS version), appears to be OK. Odd.
3. Retry download doesn't appear to work (used to get grey bits showing what was known good) now just sits there staring at me doing what it was doing or not doing, as appropriate.
Regards,
Francis.