If event_mask is commented, it defaults to "all", if you limit the event mask you will miss some events... I do not know why are you having problems, just live event_mask commented, it should work fine. Are you connecting to AMI locally? or via wan / slow link? You can always start fop2_server in debug mode, but that will make it slower, but will help you see if events are getting corrupted, or the ami connection reset.
If you are running FOP1, disable it, it will make AMI perform better without an unneeded extra connection.