Does this also affect other RRD graphs, or only those two you mentioned? Can you please check file modification times for /var/log/reporting/rrd/*.rrd ? If only those two graphs are affected, can you please also check if the sysmond process is running? ("ps aufx|grep sysmond")
Those files should be updated in regular intervals. Can you please check if there is a hanging "system-reporter.pl" process? This process should be started every 15 minutes by the cron daemon, but it should not take more than a couple of seconds to complete it's work. Just check for this process (using "ps auxf"), and if the START time is way back just kill the process.