If you use cacti and have a lot of graphs on it and suddenly some of the graphs are up-to-date while some are not, it is a good idea to check the Devices page. See if the device of which the graphs belong to is up.
I just noticed my some of my cacti graphs are 2 weeks old because SNMPD on one of the servers died, so cacti didn’t update graphs which belong to that host. It took me a while to find out what was causing the problem since cacti’s log didn’t mention anything. 🙁