The U.S. Department of Defense has 6,500 UAVs available, and with the demand in Afghanistan, Iraq and other areas of high counter-terrorism activity, there are not enough to go around. Afghanistan gets priority, and this has led to complaints from other areas where the shortage is giving local Islamic radicals a cloak of anonymity they would not have if there were more UAVs around.
Most of those 6,500 UAVs are very small (under ten pound) aircraft like the Raven. These only stay in the air for an hour per sortie, and have a short range. The UAVs that are in most demand are larger ones, like the 159 kg (350 pound) Shadow-200 and Predator class (one ton and up) vehicles.
In response, the Department of Defense is building a lot more UAVs, and will have over 8,000 within two years. Also, some of the secondary theaters (like Somalia, and other parts of Africa) are getting more UAVs, mainly by moving those withdrawn from Iraq, south. Units back in the United States, have fewer UAVs to train with. These units get more UAVs when they go overseas. But Afghanistan still has priority, and will for a while.