One of the possible causes (but by no means, the only possible cause--keep seeing different docs) is a chronic fungal infection. It can cause many sinusitis symptoms without enough congestion to show up on scans. If your ENT even knows about it, there's no treatment available yet. (Google: Dr. Jens Ponikau) He has the patent on the eventual treatment.
For now, the effective treatment for it (and yes, I got a mold/fungus infection after I cleared up all of my bacteria sinusitis problems, and cured that too) is the Flip-Turn Sinus Flush. If you've tried a regular saline irrigation, it wont work because gravity prevents saline from reaching your upper sinuses, which is where the problem usually resides (note your pain behind your eyes).
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground.
It's also rather brutal feeling at first, but it will remove anything that doesn't belong in your sinus cavities. Good luck