What color is the snot? If it is clear with an occassional or just a little yellow, could still be the cold which could last 3-4 weeks sometimes, or could be allergies. I would check and see if the CF team has any reasons why you should not try Zrytec (Nathan takes Zrytec at bedtime and Allegra in the morning and it really helps him)
If the snot is green and yellow, than it may be more than just a cold, probably not allergies but always possible, but either way she needs more antibiotics. I personally do not like bactrim for staph, we usually use doxy and get much better results, but I realize all the kids are different and it could be an age thing. Depending on age, you could also ask if you could try a inhaled antibiotic and use a mask so that it gets into the sinus's.
A doctor hear in the Denver area developed a antibiotic rinse that Nathan uses in his sinuses whenever he has any drainage at all. They mix it in the sterile saline and then you use a syringe to draw it up, attach a "MAD" (Mucusal(something) atamizer device) to shot up in the sinuses. The antibiotic they are mixing in the saline is gentamyacin, which is the inhaled antibiotic I was trying to think of that we used when Nathan was under 5 years old. Either way that you can get that gent up in the sinuses I think would be fine and would do the job. It might be worth asking about and then trying if they will agree to let you. Sometimes they are a little more willing to try something other than oral plus this is targeting the problem area.
If they need more info about the gent rinse, they can probably get the infor from the Denver Children's CF Center. If you go that route and try that, make sure that the pharmacy DOES NOT mix it in the ocean spary and gives you that, it kind of defeats the purpose using a bottle that you squeeze then sucks the crud out of her nose into the bottle. If you can get a MAD that is helpful but you can just draw 3cc's in a syringe, take the needle off, and squirt the entire 3cc's up one side and then another 3 cc's up the other side and it works just fine, the MAD makes it a little easier, but not totally necessary. I would do the rinses if she has any kind of color in the snot at all personally along with an oral if needed.
Sorry, this is probably more than you wanted to know or were asking for.