You need to have a complete immunological workup.  You might be one of
the unfortunate few who has a congenital immunological deficiency.  If
you do, then prednisone will make it even harder for your weakened
immune system to cope with the infection:

"Any patients with chronic sinusitis which is poorly responsive to
treatment should have an extensive immunological evaluation including
immunoglobulin levels, possibly IgG subtypes, and antibody testing.
Pneumococcal, diphtheria, and tetanus antibodies should be tested before
and after Pneumovax and diphtheria/tetanus immunizations. Evaluation
should be done by an immunologist familiar with testing as results can
be difficult to interpret. Briefly, blood tests should be done testing
the response to 23 different subtypes of the pneumococcal bacteria.
There should be an adequate rise in the antibody titer measured by the
blood test to determine that the patient has responded to the vaccine.
In cases where an immunodeficiency is found, monthly immunoglobulin
(IVIG) therapy may need to be initiated. Selected patients may
occasionally warrant IVIG despite normal antibody levels. Such patients
must be selectively chosen by experienced clinicians, as IVIG is
extremely costly, especially since insurance is often reluctant to pay
for such an expensive treatment."

http://www.sinuses.com/postsurg.htm

Depending on your personal sexual history, you should also be tested for
HIV.

Finally, my ENT and a few others (e.g. at the Cleveland Clinic) are
starting to realize that gastroesophageal reflux (GERD) can worsen
sinusitis, by refluxing stomach contents all the way up into the
nasopharynx.  Anti-acid medications (Prevacid, Nexium) will NOT correct
this problem, since they only prevent acid but not the physical
regurgitation.  My ENT has told me about some patients who have
experienced near-cures by having Nissen fundoplication surgeries to
tighten their Lower Esophageal Sphincters.  For this you would need to
find an ENT and a gastroenterologist who are willing to work together;
many gastroenterologists are skeptical of this theory and won't do the
surgery just to try to fix sinusitis.

Good luck.