It is difficult to determine the exact reasons for the lack of response you are describing. This could be because the sinuses are totally blocked and are acting as an abscess, which needs to be drained. It could be that the infection is not responding to the antibiotic that is being prescribed. It is possible, but not as likely, that this is a problem with your immune system. I would suggest seeing an infectious disease specialist to determine how they would approach such a resistant infection.