Combat is a dynamic process, with both sides constantly adapting to each others tactics. Such is the case with the American use of snipers in Iraq. During the first two years of the war, U.S. snipers were each getting several kills a month. Now, most get less than one a month. The easy targets are all dead, and the survivors have adapted. The enemy now knows what American snipers can do, and operate much more carefully when American troops are around. The enemy also knows what the American ROE (Rules of Engagement) are, and one of these rules stipulates that snipers can only fire on armed hostiles. So the enemy do not carry weapons openly until they actually have to use them. The enemy also uses human shields more frequently. Some times the civilian shields are paid, often they are just terrorized into complying.
Civilians are rewarded if they find where an American sniper is hiding, and report the location. This reward program has been a major problem for U.S. snipers, because it has turned into a major activity for kids in combat zones. Find a sniper, make a large sum (for a poor Iraqi) of money.
But by keeping the sniper kills down, the enemy has also had to limit their movement, and the kinds of operations they can carry out. A few teams of snipers can keep several kilometers of highway free of roadside bombs, or keep enemy fighters out of certain neighborhoods. The enemy has adapted, and so have the Americans, who now often use their snipers just to scare the enemy away.