If a home has brick, it’s especially important that the bricklayer correctly place a row lock underneath the windows.

Some installers will use the bottom of the window as their straight edge as they lay the bricks across, leaving no room for the home's dimensional lumber to shrink. When this happens, the wood moves but the brick won’t budge. Eventually, something has to give and that “something” is the window.

In order to prevent a hump from forming in the sill, bricklayers should leave a minimum of a 3/8^th inch gap all the way across the rowlock. Don’t allow the brick to touch the window. This leaves room for the structure to shrink. 

Here, a spacer is used for that purpose.