Force a verbose logrotate run:
sudo logrotate -fv /etc/logrotate.conf
/var/log/syslog is not being rotated.
A forced run of logrotate shows the following for syslog:
rotating pattern: /var/log/syslog forced from command line (7 rotations) empty log files are not rotated, old logs are removed considering log /var/log/syslog log needs rotating rotating log /var/log/syslog, log->rotateCount is 7 dateext suffix '-20160630' glob pattern '-[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]' compressing log with: /bin/gzip error: error creating output file /var/log/syslog.1.gz: File exists log /var/log/syslog.8.gz doesn't exist -- won't try to dispose of it
Check to see if /var/log/syslog.1.gz is 0 KB.
If so, delete it and syslog rotation should now work.
Source: Raspberry Pi Stack Exchange - raspbian - Why are system logs not rotating?
A forced run of logrotate shows:
error: /etc/logrotate.conf:35 unknown group 'group1'
And:
error: found error in /var/log/custom.log , skipping
Check if a 'create' is used in the logrotate configuration:
create 0664 user1 group1
And check if both the user and group exist, and modify the create to use a user and group that exists. (In this case the group did not exist.)