FxM (Foglight Experience Monitor) Alarm set to detect crashed servers not firing when Server traffic volume goes to 0. In the table view, those time periods with no data show up as zeros. (not "na" or "null" or something like that). So putting in Alarm Definition logic like "fire alarm if number of Page Downloads < 1" ought to work.
This functionality is currently not possible. Those zeros are see in the charts or table are not really a zero--that the real number wouldn't make any sense (something related to division by zero problems). So to avoid having those entries, FxM covers them up with the zeros, as a label. But mathematical calculations can't be done on those zeros.
This has been logged as enhancement request BR1737