In Applications Manager's MS SQL monitor, metrics under Database tab are collected once in an hour by default. Therefore, under Admin -> Performance polling -> MS SQL -> Database Metrics, the option 'Collect SQL Database metrics once in an hour' is enabled by default. If this option is enabled, then the thresholds will be applied only once in an hour and the alerts will work based on that.
You can disable this checkbox in case you want to collect data for every poll.