I agree with asking your doctor about allergies. Do you live in a city? Hot summers make the pollution levels in the air go sky high. Also, if you live in the country near a field pollen and dust from the fields can make you very stuffy. I had that problem growing up in Ohio.
I also had a friend who had polyps on her sinuses that made her nose stuffy 24/7. She had to have them removed to clear her nasal passge out.
Nasal sprays are only a temporary solution. Like the previous poster said, eventually they stop working and can make things worse. See your doc.