I would suggest you compare via HMC the settings of the virtual ethernet adapter of one of your working LPARs with the non-working one.
Both have to be in a VLAN that is set on the SEA on the VIO. Your SEA can have more than 1 Virtual Lan ID. It needs to match at least one VLAN-ID between your LPAR's vAdapter and the VIO/SEA. It could also be, that all your client LPAR vAdapters are in the same VLAN which makes things a tad easier when comparing settings.
Maybe you have accidentally hooked a box for trunking or priority, whatever.
Have no HMC at hand, but here is a link, that shows exactly how things are set up:
http://www.ibm.com/developerworks/sy...5-virtualvlan/