Depends on version, in R80.10 and earlier NetFlow data was collected and sent by SecureXL (sim driver).
In R80.20 and later this function was moved into the Firewall Worker/Instance, and thus requires Accounting to be set on any rules for which you want NetFlow data. I believe the NetFlow data is handed off by the Firewall Worker/Instance to the fwd daemon for transmission to the Collector, try checking the $FWDIR/log/fwd.elg file for any error messages around the time NetFlow stopped working. If NetFlow stops working again, instead of rebooting try restarting the fwd daemon (this will not cause an outage, but will cause a cluster failover).
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