The answer is that they can't.
The Nazgul already tried to sniff out Frodo and the Ring in the Shire, and it doesn't work as easily as this question assumes; the first time described in Three is Company:
When it reached the tree and was level with Frodo the horse stopped. The riding figure sat quite still with its head bowed, as if listening. From inside the hood came a noise as of someone sniffing to catch an elusive scent; the head turned from side to side of the road ... At that moment the rider sat up, and shook the reins. The horse stepped forward, walking slowly at first, and then breaking into a quick trot.
And the second time:
As Frodo watched he saw something dark pass across the lighter space between two trees, and then halt. It looked like the black shade of a horse led by a smaller black shadow. The black shadow stood close to the point where they had left the path, and it swayed from side to side. Frodo thought he heard the sound of snuffling. The shadow bent to the ground, and then began to crawl towards him.
The second one in particular is described in the same manner as if it were a dog following a scent.
The observations about a Nazgul's sense of smell are therefore:
- The range of the sense is limited. The first time the Nazgul fails to pick up the scent.
- Catching the scent requires some concentration and effort:
- The first time the horse stops and the Nazgul keeps still while sniffing.
- The second time the Nazgul is dismounted and needs to cast around a bit before it gets the scent.
- A scent, once caught, needs to be followed. Catching the scent is not enough to tell you where the quarry is, what direction you need to go, etc.
Now consider a winged Nazgul hurtling over the Dead Marshes. There is going to be neither time nor opportunity to do any of these.