I have a Birmese cat who we have had for a year now.
When we got him he caught a cold from our other cat who had recently been to a vet clinic.
Our vet put him on antibiotics, however this was only a temporary relief. And for the last year he has been back and forth from the clinic to get more antibiotics, and also to have his nose checked for foreign bodies, which was negative.
It seems to only affect one nostril and the antibiotics helps for a while, but we think his growth has been stunted and he gets quite uncomfortable when its blocked up. Any ideas?

Make sure your vet does a fungal culture on the effected side of the nose.

Also, if your cat has clamydia, this could be a re-occuring thing for the rest of the cat's life.

Check into using herbal immune system enhancers such as echinacea. That might help whatever the problem happens to be.

Good luck!