Please set LongRunningQueryThreshold server configuration parameter to, e.g. 10000. If queries that take longer then 10 second would happen, they will be written to server log file.

If not responding events are happening, but no log running queries are logged, then it's probably locks in server process.
If your server is on Linux and can catch the moment, when this event happens, try running this script:
https://github.com/netxms/netxms/blob/master/tools/capture_netxmsd_threads.sh

gdb should be installed on the system to run this script. Script should be run 3-4 times with interval 30-60 seconds. It will create files with debug information in /tmp. Please share these files.

It's also possible to run this script as action for not responding event, but care should be taken not to run this script too often, as it's quite heavy temporarily pauses all netxms processes. So the action should have a script that stores last launch time in, e.g. persistent storage and checks that some time passed since that time.