Why does a fart sometimes smell and sometimes not?

What makes a fart smell is usually methane and other gasses that have
foul smells. Different people produce different amounts of these
gasses in their intestines, so their farts will smell differently.

As an experienced farter, I would say that it has a lot to do with
what you have been eating.

http://www.heptune.com/farts.html

Other gases, vapors and particulates cause the oder. Methane is
usually present, but is oderless. The chemistry in the intestines
varies from hour to hour depending on the food type, water and air
ratio we swallow, and various feedback systems in the human body. Some
of the oders are excreations of micro organisms that live in our
digestive track, some of which are helpful in getting proper
nuetrition from our food. Hydrogen sulfide has a strong oder, and is
occasionally the principle cause of the stink.  Neil

My theory is that the smell is caused by subatomic farticles.

It may sound fartastic but your theory stinks. :p