Because flows are sent UDP where no windowing is being used, packets can be dropped silently. In such cases, JSA can track missing packets by the sequence numbers which is partially how the packets are stitched together to form the flows.

There is a setting in the flow source configuration that enables/disables the sequence number tracking that is on by default. In this case, when a packet is not received (dropped, never sent, network disruption, 3rd party between blocks, etc) JSA will notify in qradar.log under the qradar process (typically qflow0) which sequence numbers are missing. 

The configuration is defined in page 158 of the User Guide "Configure a Flow Processor":
"https://www.juniper.net/documentation/us/en/software/jsa7.5.0/jsa/jsa.pdf