Normally scheduled triggers are executed only for open tickets. If you need scheduled triggers to execute also for closed there is a setting in database to specify time period (time from current time) for tickets to be executed.
Note: Enabling scheduled triggers for closed tickets for long time period could cause bad system performance. System have to regularly scan large amounts of tickets.
If all closed ticket should be affected to set some value in some field, than it would be better to run a script on Db level first and then activate the scheduled trigger on DB and define the trigger in NSP.
If you need this function, contact NSP Support (nspsupport@nilex.se).