Hello everyone! Mommies, can you tell me how to treat long-lasting phl…

Hello everyone! Mommies, can you tell me how to treat long-lasting phlegm? My 1.7 year old has been having thick phlegm for three weeks now, I clean it up two to three times a day with saline spray and all that. Inhalation with a physical solution doesn't help (usually helped quickly, and then took the drops of Nazivin for the first week, then stopped).