This is because the nasal mucus is blocking the nasal passage, meaning that if there is anything that he/she is trying to smell will be blocked by the mucus and will not be able to go down into the nasal passage where the smells are detected.
If you have a runny nose then the mucus will block the path that smell goes through. So if you are trying to smell a flower then you will have a hard time because the mucus is in the way, or blocking the entrance path. Hope this helps:)
