There must be error messages in the log, prior this last message.
Click on the nzb in history, then on button Log, save log into file. Post the log to pastebin.com and a link here.
An alternative approach would be to use command line interface which can manipulate files using regular expressions. The following command resumes all .jpg-files in nzb mynzb: nzbget -L -E FR U "mynzb/.*\.jpg" But be very careful with the last parameter which is a regular expression in POSIX Extende...
If your goal is to automate then you need to write a script, more specifically a queue script . In that script you use api method listfiles to get content of the nzb. Then find the ids of files you want to resume and use method editqueue with command FileResume . As an example of queue script see Qu...
If both nzb-files are still in queue you can select them and then use command "merge".
If the first is already downloaded (and failed) you can download additional pars separately, move files into the directory of first nzb and then use command "Post-process again" in history.