I have this problem due to a blocked nostril - it just seems to be a bit narrower than the other one and therefore it doesn't take much to block it.
I am going in for an operation in a couple of months to widen the passages by basically burning away surplus flesh. I had a similar thing done a while ago for sinus pain and it worked a treat. It just lets the air flow round your head more effectively and even if you are prone to allergies it takes longer for things to block up as the pipes are wider. Hopefully it will work :-)
Hufty.