This article explains why traffic sampling may not be working for interfaces within a routing instance in J-Flow Version 5.

In J-Flow version 5, traffic sampling will not work for interfaces that are part of the routing instance.

J-Flow version 5 has a limitation and does not support traffic sampling for interfaces within a routing instance. To enable traffic sampling for interfaces in a routing instance, J-Flow version 9 must be configured. This applies even when the J-Flow server remains reachable through the default routing instance.

For a similar setup where J-Flow is accessible via the default routing instance while the traffic interface belongs to a non-default routing instance, refer to the following document: [SRX] How to configure SRX high end chassis cluster J-flow version 9 when traffic interfaces are in a routing instance or Flow Collector is reachable via routing instance only