First, the its no longer a wartime situation. This means American troops dont go charging towards objectives in large numbers, with artillery and bombers ready to obliterate any opposition. In this wartime mode, American troops moved fast and shot frequently at any sign of resistance. A year later, most of the firepower is gone, and the troops move more slowly to avoid civilian casualties.
The Rules of Engagement (ROE) U.S. troops operate by put great emphasis on avoiding civilian casualties. While the ROE put the lives of U.S. troops first, the need to be more careful slows things down in a way that makes American troops more vulnerable.
Lastly, the Iraqis, like the Afghans in 2001, quickly learned how to cope with some of the American battlefield advantages. In particular, the Iraqis learned to exploit the ROE. This meant setting up combat bases in mosques and moving and firing while surrounded by women and children. The civilians usually were not volunteers for this duty, but Iraqis with guns tend to get their way with other Iraqis.
After over a week of negotiation, the local leadership in Fallujah agreed to a deal that would surrender all the illegal weapons held in the city and allow joint (U.S./Iraqi) patrols in the city. There would also be Iraqi cooperation in finding those guilty of murder and other crimes.
The al Sadr militias remain disbanded or contained, with al Sadr himself surrounded in a mosque in Najaf and, in effect, under siege by more senior Shia clerics who are demanding that he surrender.
So far in April, 99 U.S. troops have been killed in Iraq. There are three reasons for that high number of combat losses.