Sinus X-rays (as opposed to CT scans) are known to have a high
percentage of false negatives; they often fail to find pockets of
infection.  That's why CT scans have become the diagnostic tool of
choice for sinusitis.

So you might have a continuing low-level sinus infection even with a
normal-looking sinus X-ray.

Zithromax, an antibiotic in the macrolide class, is often a good
second-line antibiotic and is worth a try.  However, macrolides seem to
be ineffective against sinus infections caused by Staph aureus and if
you've got one of those, then you may have to go with a quinolone like
Avelox.

The biggest problem with Zithromax is the dosing.  The manufacturer
claims that because of its long half-life, you only need to take 5 days
worth (the infamous "Z-pak" package it comes in).  That is simply false
for stubborn sinus infections.  Finding a doctor willing to prescribe 2
or 3 weeks worth of Zithromax has been a problem for me.

Mel,

Until you use something powerful enough to get rid of the bug that's
bugging you, you won't get rid of it.  It'll just lie dormant, and
it's been my experience that it comes back worse the next time.

I would stay away from Zithromax.  I'd rather take nothing than take
that.  I guess doctors still prescribe it, but I'll never ever take
it.  If you don't want to take something as strong as Levaquin, or to
a lesser extent, Biaxin XL, you could ask your doc about Cipro, and if
you live in Texas or Florida or somewhere, you can get the generic
form of it for $4 at Wal-Mart.  Cipro has worked for me in the past.
But sometimes it hasn't, so I don't ask for it anymore.  

You had x-rays.  But you didn't have a CT scan, which seems

In any case, make sure you ask your doctor for AT LEAST a 14 day
supply, and make sure you take every last bit of it, even on the last
few days when you feel considerably better.  Irrigate also, and if
you're congested, try a Sudafed-type decongestant, or Musinex to try
to get phlegm up.

Is it a regular GP you're going to?  One of them told me that the
recommendation was that only 7 days worth of antibiotics is necessary.
They can't know everything, I guess.   I can tell you from experience,
they really don't know what to do when it comes to a chronic sinus
problem that lasts as long as yours.  A good ENT might put you on a
longer course of antibiotics.  At least that's been my experience.
Good luck and report back to us as to what your doc said.

Rick