Check if the permissions are set properly

If your graphs are not updating anymore, if some of them don't display, or if the HTML pages don't get created at all, it is highly likely that your problem is related to permissions. Remember: you are never, ever supposed to run munin as root. If you do so, even only once, it will create all the rrd, html, png files with the owner root; therefore, munin, as run by the cronjob as user munin, won't be available to write to them anymore. So, check the rights on the files in /var/www/munin and in /var/lib/munin - these directories and all the files they contain should be owned and be writeable by the user munin. /var/www/munin should even by writeable by the HTTP process. The user munin should also have read access on the files in /etc/munin/ (and subdirectories), and read and execute rights on the files in /usr/share/munin.

Have a look at the log files

If you believe the rights are correct, you should read the log files, which are in /var/log/munin. If a node appears in the list but contains no data, maybe your munin server is failing to connect to the node; you should read munin-update.log. Munin-graph.log and munin-html.log may also help you track down wrong permissions.

Connect to the failing node manually

If you still don't find the error or need more information, try connecting from the server to the faulty node with telnet as described previously, and fetch the data manually. You may get a detailled error message from the plugin.

Hint: You can also at every moment delete all the files in /var/www/munin: they will be rebuilt after the next time munin is started by cron. You won't lose any data, as the data is stored in rrd files in another directory (/var/lib/munin).