There's a priority system in place that goes from the center pylon outwards. That means that if you want to only jettison your inboard stores after you've already jettisoned your center pylon, the center pylon will have to be switched back to the OFF position on the ARMT POSITION SELECT panel, before you can jettison the inboard ones. Later, if you want to jettison the outboard ones, the inboard and center pylons will have to be OFF, etc.
If the center pylon is still attached, you can of course jettison the inboard pylons straight away.
Also, if the center pylon is still attached and you've jettisoned the inboard stores already, you could leave the inboard pylons in the ON position and then jettison the center pylon, because of the priority circuitry.
However, if you wanted to jettison the outboard pylons instead, nothing would happen, because the trigger would still go to the inboard pylons.
Hope that makes some sense.