well i'm still using B14 as i can't download B15, i always have an error like invalid file or can't read the file ...
anyway, i often dowload pictures and most of the time i have :
1- files in the failed list when they are just downloaded (and i don't try to redownload maybe due to point 3?)
2- failed files without reason because never downloaded before
3- and sometimes i see the files beeing reassigned at the bottom of the download list AND appearing in the failed list at same times.