Hi Guys -
When people choose to configure subinterfaces on an ASA, as I generally do, they commonly don't configure the physical interface at all -- no NAMEIF, no IP address, etc. In this case you'll need to put a 'NAMEIF' on the physical interface in order to apply the priority queue, which will then be inherited by the subinterfaces as described above.
Note that the service-policy (that specifies "priority" queueing for some traffic) is still applied to the subinterface.
description [OUTSIDE] MAIN OUTSIDE INTERFACE
no ip address
description OUTSIDE INTERFACE
ip address 18.104.22.168 255.255.255.248 standby 22.214.171.124
service-policy QOS_OUTSIDE interface OUTSIDE