I would take her to an ENT specialist. Sometimes the pedi's don't have the right equipment to diagnose these sort of problems. My three year old has been having "sinus infection symptoms", my pedi diagnosed it as a cold. I didn't feel like that was the case because he also had horrible breath. I brought him to the ENT and they looked at his adenoids with a very simple camera and they were VERY enlarged and infected. With the adenoid problems one of the signs can be constant congestion. After multiple infections my ENT decided that it's time to remove them. I am currently awaiting a surgery date. My five year old had his adenoids out and after dealing with multiple "sinus infections" and CONSTANT stuffiness for so long, once he had them removed he is by far the healthiest kid! He gets a cold MAYBE twice a year and it lasts for 2 days tops! I am not saying that this is your daughter's case, but I would seriously consider seeking a second opinion with an ENT. Good luck. I wish you were in my area because I would recommend my son's ENT, he's awesome.
**Just to add...
My husband has had chronic sinusitis, after having an MRI it was found that it was caused by his sinuses being WAY too narrow. All of the immune strengthing products on earth would not have helped. Strengthing the immune system is always a good idea, but it isn't always the answer to a problem. Neither is a chiropractor. Although both ideas can be great for certain situations, they may not be for this one. My advice would be to have her thoroughly checked by an ENT and if her upper respiratory system is functioning and developed normally then I would move on to the other suggestions, but not until you know that her sinuses, adenoids and eustacian tubes are in working order.