Most healthy snakes have no problem defecating. There are many factors involved in this though, including hydration and exercise. Some of the arboreal species, which have slower natural metabolisms, and higher humidity needs, can also become very complaisant when all of their other needs are met, meaning they don't move around as much. This can actually cause problems with any species. Soaking most reptiles in lukewarm water helps them defecate, so that is the main practice recommended if there are problems.
As far as acting hungry, that truly depends on the exact species once again. Most snakes are opportunistic predators, and will eat food whenever it presents itself, because they may not eat again for months. I don't think they experience hunger in any way that we would recognize. So a healthy captive snake may appear "hungry", or in hunting mode, right after eating. Because that is natural for them. Eventually, some snakes will learn that you are going to feed them again in 7 days, 10 days, 14 days, whatever, and get used to that schedule.
