This is probably due to your user account not being listed in all of the required groups in /usr/local/nagios/etc/cgi.cfg.
One of my colleagues had the same problem recently. All of us were able to see all the graphics, but he wasn't. We added him to all of the "allow" groups in cgi.cfg and the problem was gone.
I meant the "authorized for" entries, not the "allow" entries.