Silva,
Is the Device Up alarm enabled? To check (in NetSIght OneView or the browser based version), go to Alarms and Events, Alarm Configuration, and make sure there is a check next to Device Up.
If there is, the next thing to check is if the Device Down has Device Up as the clear condition. To check this, double click on Device Down, go to Other Options tab, and verified Cleared by Alarms is enabled and Device Up is selected in dropdown.
Jeff