Sorry the GMs have said pretty consistently that they cannot grant missed encounter credits, regardless of the circumstances. Just jump on the next Sidi run and you should be able to get the kill credit you need. If you already have the feathers and just want to buy an apoc-credit item, you can advertise in /trade for someone to swap feathers with you.
Have you been close/in range to the boss? Did you die out of range? Did you (or someone) in your grp do damage? Just being in the BG without doing any damage/healing wont be enough. Also you have to be close to the boss. Standing somewhere else in the dungeon or at the entrance wont be enough either.
In most cases where players didnt get the credit one or more of the above mentioned possibilities have been the reason for not getting a credit.