load balancing on fab members

Dears , 

 I have SRX5800 cluster with two xe members on the fab , traffic flow only on one xe and the other is not used : 

Logical interface fab1.0 (Index 71) (SNMP ifIndex 543) (Generation 168)
Link:

    Statistics Packets        pps      Bytes           bps
   xe-14/0/0.0
     Input : 0 0 0 0
     Output: 155255270634   9796    74446407842625   37758496
   xe-21/0/0.0
     Input : 0 0 0 0
     Output: 7828568159561   687435 9161592098197963 6722743904

AFIK it should work as an aggregation right ?  i have this behavior in other cluster too with same Junos version 12.1X46