Problem nagios checks

Hi,
i have installed nagios 3.2.3 and configured it to check some processes on our servers. recently, i noticed that some of the checks are stopped and some others are still done…when stopping/starting the nagios server, all the checks are back for some hours and then some of them (randomly) stop…even in the log files, i didn’t find the cause of that issue…did somebody have an idea about that?