6 month old congested?

My baby has sounded really snotty the last few weeks. It just won't go away. We run a cool mist humidifier in her room, use saline nasel drops & suction her nose. Nothing comes out but I can hear it up in her sinuses. She's not running a fever & she doesn't act like it bothers her but I'm a little concerned. Is there anything else I can do? Is it a sinus infection or just allergies?