sql server UPDATE TRIGGER doesn't fire - sql-server-2008

Question

i have a trigger like below, the logic is to change FID status after fidRule status changed.
in my app, i update 1 row in each statement,
but i found sometimes(very rare) the trigger not firing.
ALTER TRIGGER [dbo].[triggerSetFIDStatus]
ON [dbo].[FIDRules]
AFTER UPDATE
AS
BEGIN
set nocount on
DECLARE #ruleStatus INT
DECLARE #newRuleStatus INT
DECLARE #FIDAlertStatus INT
DECLARE #FIDId INT
DECLARE #isFIDEnabled int
DECLARE #ruleId INT
SELECT #ruleStatus = deleted.alertStatus,
#FIDId = deleted.FIDID,
#ruleId = deleted.id
from
deleted
SELECT #newRuleStatus = inserted.alertStatus
from
inserted
SELECT #FIDAlertStatus = alertStatus,
#isFIDEnabled= isEnabled
FROM FID
WHERE id = #FIDId
IF #FIDAlertStatus <> #newRuleStatus
BEGIN
-- change FID-status by FIDRule-status
UPDATE [dbo].[FID] SET alertStatus=#newRuleStatus WHERE id=#FIDId
END
IF #newRuleStatus >= 0 AND #newRuleStatus <> #ruleStatus
UPDATE [dbo].[FIDRules] SET isAlertStatChanged=1, AlertStatChangeTime = SYSUTCDATETIME() WHERE id=#ruleId
END

Answer 1

The trigger will not be fired if the UPDATE statement fails or another triggers fails to execute before this trigger is fired.
One comment about your trigger itself:
You are expecting one records from DELETED which is not always correct.
Please make your trigger robust enough in case DELETED contains multiple records
-- What if deleted contains multiple records?
SELECT #ruleStatus = deleted.alertStatus,
#FIDId = deleted.FIDID,
#ruleId = deleted.id
FROM
deleted
You can either use SELECT TOP(1) or make sure your trigger is able to handle multiple records from the DELETED list.