This was experienced by somebody else in this not well documented old thread.
The tool to brute force passwords work properly when a zip file is created from the command line. Here is a test result for a file named testfile.txt encrypted using password 'pass'.
kannamma@dink:/media/kannamma/drives/PC/TestRun$ zip --password pass locked.zip testfile.txt
adding: testfile.txt (deflated 13%)
kannamma@dink:/media/kannamma/drives/PC/TestRun$ fcrackzip -b -c 'a' -l 4 -u -v locked.zip
found file 'testfile.txt', (size cp/uc 84/ 83, flags 9, chk be51)
PASSWORD FOUND!!!!: pw == pass
kannamma@dink:/media/kannamma/drives/PC/TestRun$
However, instead of the command line, if the file is encrypted using Gnome archive manager, fcrackzip is unable to find the password. Here is the test result.
kannamma@dink:/media/kannamma/drives/PC/TestRun$ fcrackzip -b -c 'a' -l 4 -u -v lockedwithGnome.zip
found file 'testfile.txt', (size cp/uc 92/ 83, flags 1, chk 0000)
kannamma@dink:/media/kannamma/drives/PCrack/TestRun$
Why fcrack zip is incompatible with Gnome Archive Manager. What difference does it make to the file if it is encrypted using command line instead of Gnome Archive Manager?