I think sometimes it's harder for us to critique our own work because we're so close to it. If you have the luxury of no deadlines or distant deadlines, put the image you're working on away. Don't think about it, don't look at it. Then, after several days, get it out and look at it with fresh eyes. You might see things that need resolved that didn't stand out before. This also helps if, like me, you sometimes over-analyze things and think everything about the image is going wrong. Although sometimes I find that things ARE all wrong, ha ha.
Other things that help are: turning your image upside down, and/or backing away from it. A simple change of perspective sometimes brings new issues to light, particularly when it comes to tonal variation and focal points.