For some reason SARG has not generated the monthly reports for April and May 2009.
The daily reports for these months are there ok.
Could it be that the squid log files were rotated before sarg had a chance to generate the reports ?
Or did you stop the server when it should have generated the monthly reports ?
Could you also double check that all the daily reports are really valid ? If one entry in one access.log file chokes sarg, any report containing that entry will be missing. If you have a weekly report, you could check if all the weeks are there.
Do you have the monthly reports for June and Jully ?
Can you run sarg manually to generate the missing reports and see if it outputs any error message ? You can add the -z and -x flags to have a more verbose output if it is necessary.
It could be produced by a conflict in the /tmp/sarg directory used by sarg as reported by adib729 in http://sourceforge.net/projects/sarg/forums/forum/363374/topic/3401861.
By default, sarg always uses /tmp/sarg to store its temporary files but if cron starts sarg multiple times, there will be a conflict in that directory. I also experienced two missing monthly reports and my investigations lead me to that directory.
You can fix this by asking sarg to use another directory with the -w option. For instance, in my cron.monthly, I use -w /tmp/sarg.monthy. In cron.weekly, I use -w /tmp/sarg.weekly, and so on.
I have to wait until the month of November to see if it really solves the problem.