I always almost completely lose the ability to smell the odor of cannabis
smoke quickly after partaking... My understanding is that this is a very common occurrence.
I think a lot of it comes down to olfactory adaptation, or the tendency to rapidly become acclimated to the odors within your immediate environment. In other words, our sense of smell just naturally starts to block out the scents surrounding us after a short period of time, most likely so it can be more effectively alerted to any new aromas.
The evolutionary advantage to this is a lot easier to understand if you look at nature: A mouse in a field of flowers could be in a world of trouble if it was unable to block out that fragrance in order to smell a predator approaching.
For the same reason, if you're burning a scented candle or incense, burn some food while cooking, or have been puffing on a joint, after awhile you pretty much stop smelling the particular odor in question until you leave the area and then come back. Because your mind is purposefully blocking it out in order to try and pick up on new smells, you get used to the smell quickly and it becomes much more difficult to recognize it.