Depending on the e-liquid you have in your tank and the size of the holes in the cartomizer, it may not be wicking enough to keep it saturated.
When this happens, you will get dry, bad tasting, or muted hits. Also it will tend to shorten the life cycle of the carto, since it is essentially burning itself and the filler, not the juice.
A member on here baditude has a great blog
http://www.e-cigarette-forum.com/forum/blogs/baditude/4830-8-cartotank-help-noob.htmlabout cartos
Topping off, basically helps keep the filler saturated.