Alert on Low disk space

Hi  All

I have created an alert of WMI event in sql server  which send mail if disk space on D drive is less than 10GB

Query :

-----------------------------------

SELECT * from __InstanceModificationEvent
WITHIN 60
WHERE
TargetInstance ISA 'CIM_LogicalDisk'
AND
TargetInstance.FreeSpace < 10000000000
AND
TargetInstance.DeviceID = 'D:'

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when I enable this alert It sends alert for low disk space

but some time it does'nt send alerts. I am not sure when this __InstanceModificationEvent   occure

and is there any another way to alert low disk space in my mailbox