ESM usually cleans 5-10%(depending on build version) every 5 minutes. If it records more than that percentage within 5 minutes, the Delete Threshold process cannot run. This is an automatic clean up process controlled by the ipConfigure Recording Control service.
In order to resolve this issue, you can manually delete some of the oldest video to create room for the delete process to run. You can also disable the cameras and restart the ipConfigure Recording Control process in order to give the Threshold Delete Process enough resources.
Also to prevent this in the future, you may want to change the "max disk storage" in the archive volume tab in the devices tree in the ESM Admin interface to record only 90% of the disk space (instead of the default 0 which uses the whole space.) This way it will always have that extra buffer in case it does write more than the percentage between the 5 minute clean up intervals.