Why is SWAN dimming?
SWAN seems to be on a parabolic orbit. This means that it is the first time that it is interacting with the Sun, making it a "new" comet. New comets are covered in a layer of very volatile elements that will vaporize pretty fast when the comet is still far away from the Sun. This causes a surge in brightness. But once those very volatile elements are gone, it stops getting brighter and the comet's brightness can even decrease. 

In addition to this, it is mostly losing gas, so its tail isn't as visible as if it were a tail of dust.

(From this article, in French)