difficult to diagnose daughter

My 12 yr. old daughter has been suffering with nausea, some gurgling, lots of phlegm from post-nasal drip for several months. We have seen GI and ENT. Her endoscopy was fine with no signs of reflux, but she has symptoms every day- not quite heartburn, though. she is on zantac, singulair and claritin. The ENT and GI said there are no structural problems- just excessive phlegm. No one has any answers that help her. Symptoms often worse at night- she just feels bad, and a "stuck" feeling in her throat area. We really cannot pinpoint specific foods as culprits.Already negative for celiac. this is terribly frustrating!!!