Please check ndomod and Event broker module log events from nagios.log. It will provide a debug information
Btw, please confirm your nagios.cfg has the following configuration :
The common way to use socket type of Ndoutil is unix domain socket, So you can check weather the file
nagios/var/ndo.sock is exist or not ? or execute ps -ef | grep ndo to check ndo2db is running or not ?!