Often a nebulizer breathing treatment is prescribed due to a dry hacking cough and not a wheezing symptom at all.
The med in the vial expands the lungs and helps open the airways so more air is moving.
The sterile water in the vial helps the med become vapor but it also helps the dry goo to have moisture to help it loosen and get up and out.
The allergy med is likely drying her up more and the two things together are working against each other. If they aren't working then consider what you want. Do you want the stuff up and out easier? or do you want to mucus to stop running and dry up?
When we are doing breathing treatments we are in the shower several times per day simply for the moisture and getting allergens off our bodies. We are also taking Walmart brand Tussin, which is the SAME EXACT MEDICATION as Robitussin and Mucinex but costs pennies when compared to those meds. I pay $.88 for Walmart Tussin and right next to it with the same exact med in it is the same size bottle of Mucinex for $12. Why do people think it's better because it costs more?
Anyway, I also add in some liquid Children's Sudafed that I buy from my pharmacy that is a privately owned business. They order their own medications and don't depend on some warehouse shipping them what they think they need like Walmart and Walgreens do. I asked a pharmacist at Walmart if they could order some for me and they told me it was illegal to sell it, another one told me it wasn't made anymore. I pulled out the bottle I carry in my purse and showed it to them. They looked it up and were astonished. They had no idea.
The Tussin helps the goo be more liquid, up and out. The Sudafed opens the sinuses, goo drains out and isn't sitting around dried up and getting infected. This can increase coughing though. That is the way goo gets up and out. IF her coughing is too severe then that is when I'd consider adding a half dose of antihistamine to dry our out. I'd also give something for cough.
If her cough isn't productive I'd drop the allergy med for a couple of days to see what happens. But that's me. If you have the option to go to a lung doc then make an appointment. They are specialists in that area and know what's what.