Sorry Tallulah is not feeling well, in the picture you posted she looks young, how long have you had her? Airsacculitis is an inflammatory condition of the air sacs, which can be due to a bacterial infection. I am not familiar with the medicine you have been given, how are you giving it, is it directly into the mouth? Since the vet you saw admittedly does not usually see small birds it may be best to try and find a vet that has more experience with all sizes of birds. Watch your other birds closely to make sure that they do not also begin to show symptoms. Make sure Tallulah stays warm and when a bird is ill it is best to limit their activity so they can use their strength to heal.