Concerned about my mother's persistent cough

My mother is in her 70s and she has been getting coughs that is lasting longer than normal. Ive read up that the pollen count has been high lately but I feel it's something else. She is getting conjestion to dry cough and sometime it goes back and forth. she also gets thick mucus, mostly at night but sometime during the day as well. There is no sign of her being sick and she has tried different boxes of mucinex, which none have worked. The nasal spray doesn't do much for her. She can not smell or taste in the past month she first started the cough and tested negative on the Covid. Any thoughts?