Did you check to make sure 'released' was set to true?
MouseInfo mi = Greenfoot.getMouseInfo();
int btn = mi.getButton();
if (btn== 1)
{
System.out.println("Left mouse clicked");
if (released)
{
System.out.println("released is true")
leftClicked = true;
released = false;
}
}
if (btn == 3)
{
rightClicked = true;
}